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PHOTOPERIODISM and REPRODUCTION TIMING

© elfruler 2023

PHOTOPERIOD, CIRCADIAN RHYTHMS, and CIRCANNUAL RHYTHMS

Most viewers of Bald Eagle nest cams are aware of how important photoperiod, circadian rhythms, and circannual rhythms are in the lives of birds. They are fundamental to how eagles’ behaviors and physiological processes engage with the environment around them. This article provides information about what these systems are and how they work, and it explores some of the complexities of how they govern the processes of the Life History of Bald Eagles.

The photoperiod is the timespan of sunlight within a 24-hour day, usually called daytime or daylight. Its counterpart is the scotoperiod, the timespan of darkness, or nighttime. As everyone knows, the photoperiod and scotoperiod change from one day to the next following an annual cycle, and they also vary by latitude (location on the earth north or south of the equator). Changes in photoperiod and the seasons of the year are caused by the Earth’s tilt on its axis by about 23.5°and its revolution around the sun every 365.4 days. In the Northern Hemisphere during the winter months the North Pole tilts away from the sun, making the sun lower in the sky and the daytimes short. During summer the North Pole tilts toward the sun, which thus is higher in the sky and the daytimes are long.

    • The photoperiod is shortest on the Winter Solstice (~ December 21) (Lat. sol = sun + status = standing still) when the sun is lowest in the sky.
    • The photoperiod slowly increases through the Vernal (Spring) Equinox (~ March 20) (Lat. equus = equal + nox = night) when the Earth’s axis is tilted neither toward nor away from the sun, and day and night are of roughly equal duration.
    • The photoperiod continues to increase until it reaches its maximum at the Summer Solstice (~ June 20) when the sun is highest in the sky.
    • It then slowly decreases through the Autumnal Equinox (~ September 22) when the axis again is not tilted in relation to the sun, and day and night are of roughly equal duration.

This progression is mirrored in the annual cycle—or as ornithologists call it the Life History—of most birds, including Bald Eagles. Each species has its own Life History, which is part of its genetic make-up, a long-term adaptation to ensure its survival. (See more discussion of Life History here.) The central Life-History stage for birds is Reproduction, which obviously is necessary for all species’ survival. This is followed by Molt, the regular replacement of feathers, which also is essential. Many species then go on Migration, or, as I prefer to call it in reference to Bald Eagles, a post-breeding stage of Movement. Here is a schematic diagram of the Life-History stages of Bald Eagles, mapped onto 52 weeks of the year, showing the typical durations of each stage. Notice that Molt overlaps with Reproduction at the beginning and with Movement at the end (click on the image for an enlarged view):

The Life-History stages for a species revolve around the time of the year when optimal food resources are readily available to feed growing chicks as well as the parents who nurture them. Scientists refer to food availability as the ultimate factor that is critical to the timing of a species’ Reproduction. This, of course, is closely linked with the changes of seasons caused by the yearly movements of the Earth. Birds are able to “read” the seasons so that they move through the Life-History stages in such a way as to optimize their breeding success. They do this by means of internal rhythms (endogenous), biological clocks throughout an organism’s cells (which all organisms have) and that are correlated with the external rhythms (exogenous) of the changing photoperiod.

    • Circannual rhythms ( circa = about and annum = year) are months-long cycles that govern a bird’s Life History.
    • Circadian rhythms (Lat. circa = about + dies = day) are daily oscillations between high and low physiological activity.

Circannual rhythms and circadian rhythms are built into a bird’s DNA, like its Life History. The rhythms are maintained without the stimulation of conditions from the environment. Scientists have learned this by placing birds in a laboratory, maintaining constant light to simulate an unchanging photoperiod, and measuring the pattern of rhythms of their physiological systems. These endogenous rhythms are referred to as free-running.

    • Free-running circannual rhythms vary among bird species from 9 to 13 months. Even birds in tropical regions, where changes of season are minimal or non-existent, display an endogenous annual cycle.
    • Free-running circadian rhythm among birds recurs about every 23-25 hours, depending on the species.

But wild birds do not live in the artificial environment of a laboratory, and their endogenous rhythms are synchronized with and controlled by exogenous systems of the natural world, the daily and annual cycles of the photoperiod. This synchronization is referred to as entrainment. The regular and entirely predictable changes in the photoperiod over the year entrain the endogenous circannual rhythms to the exogenous rhythm of the seasons, so a species’ Life History is repeated within the 365-day timeframe of the Earth’s revolution around the sun. And the endogenous biological clock is entrained to the exogenous changes in the photoperiod over the 24 hours of the Earth’s rotation on its axis.  (Scientists sometimes refer to the photoperiod as a Zeitgeber (Ger. Zeit = time + Geber = giver) because of its role in entraining an organism’s circadian rhythms to the Earth’s exogenous rhythms. This compound German word was invented in the 20th century by Jürgen Aschoff (1951), who was a chronobiologist, a scientist who studies biological rhythms.)

HOW DOES PHOTOPERIODISM WORK IN BIRDS?

Photoperiodism refers to how an organism responds to the changing photoperiod. The mechanism begins in the brain. The Pineal Gland, the Hypothalamus, and in some species the Retinas of the eyes serve as a bridge between the exogenous world and the endogenous systems of the body. In addition to performing their other functions within the nervous system, these organs are the body’s principal internal oscillators or pacemakers, interacting with each other to maintain a body’s circadian rhythm. The pacemakers also receive information from the external environment by means of photosensitive cells, or photoreceptors. These cells are active during the daylight hours and perceive the presence of sunlight and its intensity and measure the daily photoperiod. They also retain a memory of sunlight changes over the course of the day, and thus the brain can deduce the approximate time of day—early morning, mid-afternoon, etc. (Dawson, King et al. 2001)

As they measure the photoperiod, the photoreceptors stimulate the Pineal Gland to produce the hormone Melatonin (MEL). MEL is inhibited by light, so it is secreted only during the scotoperiod, increasing as daylight diminishes near sunset and decreasing as the sun rises. MEL’s daily rhythmic secretion is the means by which the body’s endogenous daily biological rhythms are synced—or entrained—to the exogenous rhythms of the photoperiod. MEL also induces physiological changes in the body. It signals the brain to decrease its activity during the dark hours, and its circulation through the bloodstream causes lower body temperature and decreased metabolism. (See more about Melatonin, the Pineal Gland, and the Hypothalamus in the article on the Endocrine System and Hormones).

Memory of the daily photoperiod persists throughout the year. The photoreceptors in the brain measure and retain memories of changes in daylight from one day to the next. Thus they can perceive whether the photoperiod is comparatively long or short, whether it is increasing or decreasing, and whether the change is gradual or rapid. The brain also responds to the intensity of sunlight—whether the sun is high overhead or hanging low in the sky (and distorted by atmospheric refraction), as dictated by the time of day and the season. The photoreceptors’ perception of the changes in the photoperiod from one day to the next enables a bird to perceive whether it is spring or fall, summer or winter, in other words to know where they are in their annual Life History and to anticipate what is likely to happen in their environment in coming weeks.

The changing photoperiod is what researchers call a proximate factor, a current or impending situation that triggers behaviorial or physiological responses. The photoperiod is the only proximate factor that is unfailingly consistent from year to year and thus can serve as a signal to the body to prepare for transition to a new Life-History stage. For most birds in the Temperate zone, including all Bald Eagles, the photoperiod is the initial predictive cue that is reliable weeks in advance as a signal to the body to begin the transition from one Life-History stage to the next. (Less predictable proximate factors, such as temperature, food supplies, weather conditions, bonding with a mate, territorial disputes, and human disturbance are discussed below).

Photoperiod affects a bird’s activities and biology throughout its annual cycle, but particularly at two pivotal times for a breeding bird: 1) at the end of the Reproduction stage when the Testes, Ovary, and Oviduct shrink (gonadal regression), and 2) at the start of the next Reproduction stage when the Gonads redevelop (gonadal recrudescence). Gonadal maturity obviously is necessary for the production of sperm and eggs, while gonadal regression precludes the possibility of reproduction during an unfavorable season of the year, and it frees up metabolic resources for Molt and Movement.

The body’s responses to the changing photoperiod follow a progression of three states for breeding birds (see more detail article on Reproduction and Hormones), which largely coincide with the Life History stages:

Photosensitivity. In this state the bird becomes sensitive to changes in daylight after a long period of insensitivity and gonadal regression, and the body slowly transitions into the season of Preparation for Reproduction. A protein (opsin) in the brain’s photoreceptor cells undergoes a chemical reaction that signals the Hypothalamus to produce a hormone (GnRH, or Gonadotropin-Releasing Hormone) which initiates a series of hormonal secretions downstream, causing the Gonads begin to regrow. The gonadal recrudescence process is slow, but the hormonal changes gradually affect the birds’ behavior. The Testes begin to produce sperm and yolk material is slowly produced in the liver for deposition in the Ovary. The pair becomes active in their breeding territory, they start to build or refurbish nests, and they engage in bonding behaviors, including copulation.

Photostimulation. As the photoperiod continues to change, the brain’s photoreceptors stimulate significantly increased secretions from the Hypothalamus and the Pituitary Gland which activate sex hormones (Testosterone, Estrogens, and Progesterone) in the Gonads. The Gonads grow rapidly and the bird enters the Maturity phase. Nests are finalized, copulation becomes more frequent, ovulation and fertilization occur, and eggs are laid (oviposition). After oviposition, different hormones induce a transition to a less sexual and more parental phase as adults care for growing nestlings.

Photorefractoriness. The birds transition into a state of insensitivity to daylight as it settles into a long period of slow daily changes. With little exogenous stimulation and a decline of reproductive hormones, gonadal regression begins and sexual activity between adults recedes as the Termination phase of Reproduction settles in. Chicks grow and ultimately leave their nests. Photorefractoriness continues through the beginning of Molt and the Movement stage of Life History when adults begin migration or other movements outside the immediate nest area. Reproduction will not begin again until Molt is complete and photorefractoriness is dissipated at the beginning of a new Reproduction season.

This schematic diagram illustrates how these photoperiodic states correspond with the annual cycle of a Bald Eagles’ Life-History stages (click on the image for an enlarged view):

Reproduction occupies well over half of the year for Bald Eagles, and the Preparation phase begins many weeks before they lay eggs. Scientists theorize that the critical predictive cue to the brain is when the daily changing photoperiod reaches a threshold of a certain length, a critical photoperiod. This triggers the dissipation of photorefractoriness and brings the Reproductive system to a state of photosensitivity, when the long slow process of gonadal recrudescence begins. Another critical photoperiod at the end of Reproduction initiates the beginning of photorefractoriness and gonadal regression. What that “threshold of a certain length” is that defines a “critical photoperiod” cannot be pinpointed to a specific day, but it occurs within a brief window of time that maximizes a bird’s likelihood of reproductive success in the weeks that follow. This timeframe differs among species, populations, latitude, and climate, and it can shift for an individual bird from year to year.

ADAPTATION, VARIABILITY, and PLASTICITY

Bald Eagles have a relatively simple Life History (in contrast, for instance, to birds who undergo more than one Molt each year), which enables them to be flexible breeders in the timing of Reproduction, Molt, and Movement, within the boundaries of the genetically established timeframe. (Wingfield 2005) They inhabit most of North America, and the Life History schedules of different populations in this enormous area vary, a type of environmental plasticity. In general the Bald Eagle breeding season occurs progressively later in the year from south to north. But geographical latitude is not the only variable. Geophysical features like coasts, mountains, and deserts can affect climatic conditions (temperature, humidity, rainfall) that shape the timing of breeding. (Dawson 2008) The schedule may depend on habitat, such as whether a pair is nesting in an urban or a rural area. Even within the same region and climate, Bald Eagles’ egg-laying schedules can differ by a month or more, probably dictated by individual genetic make-up, lineage and experiences, and Life History. And although most individuals produce eggs each year within the same timeframe of 1-3 weeks, they may adjust their schedules if necessary due to unforeseen and disruptive events.

Bald Eagles inhabit the large Temperate Climate Zone of North America, from the Sub-Tropics (north of the Tropic of Cancer at ~ 23.44° N) to the Sub-Arctic (south of the Arctic Circle at ~ 66.5° N). This map marks the Tropic of Cancer and the Arctic Circle with thick yellow lines, and I have added red dots showing the locations of Bald Eagle nests recently live-streamed on the internet (click on the image for an enlarged view).

Based on the timing of breeding activities of Bald Eagles, I have separated the Temperate zone into several regions by latitude:

    • Sub-arctic: just below the Arctic Circle, from about 60° – 50° N, including Alaska and Yukon Territories
    • Northern tier: the Northern tier between about 50° – 43° N, including Maine, Massachusetts, Michigan, Wisconsin, Minnesota, Montana, Oregon, Washington, and British Columbia
    • Middle tier: between about 43° – 35° N, including Tennessee, North Carolina, Virginia, West Virginia, Maryland, New Jersey, New York, Pennsylvania, Ohio, Indiana, Illinois, Missouri, Iowa, Colorado, Arizona, and California
    • Sub-tropics: from about 35° – 23.4° N, including Florida, South Carolina, Georgia, Louisiana, Texas, and Oklahoma

The northernmost nests on cam are those in Alaska at about 60° N, some 448 miles south of the Arctic Circle. The southernmost nest is in south Florida at about 25° N, a few degrees north of the Tropic of Cancer and about 1725 miles north of the Equator.

The different populations of breeding Bald Eagles that these regions delineate remain fairly discrete since eagles tend to return to the general area of their natal nest to establish their own territory. All Bald Eagles have the same Life Histories, but the timing and the progression of the stages are genetically adapted to the climatic conditions of each region, commensurate with timely access to food.  Populations at lower latitudes generally begin the Reproduction stage earlier in the year, while those at higher latitudes begin later, a common pattern for wildlife.

The differences among the populations are obvious in the following schematic diagram, which shows the approximate timeframes of the phases and sub-phases of the Reproduction Life-History stage for Bald Eagles from north to south as they have been observed on nest cameras, correlated with changes in photoperiod through the year. (Each Reproduction phase—Preparation, Maturity: Sexual, Maturity: Parental, and Termination—shows the observed time range among all the nests in each region, not the schedule of an individual nest or bird. Click on the image for an enlarged view.)

This diagram illustrates a striking contrast in the start of the Reproduction season, the Preparation phase of courtship and nest-building (marked by blue shading in the diagram) when photorefractoriness has dissipated and gonadal recrudescence has commenced. (While we cannot directly see the Gonads of Bald Eagles living in the wild and whether they are growing or shrinking, the eagles’ behaviors reflect the physiological changes.) The difference is whether the eagles start after the Summer Solstice or after the Winter Solstice.

    • Eagles in the three lower regions (North Temperate, Middle Temperate, and Sub-tropical) begin after the Summer Solstice, from late summer to early fall as daylight is decreasing. Note that eagles in the southernmost Sub-tropics begin only a month or so before the eagles in the North and Middle Temperate regions. This timing is typical of most birds in the Temperate Zone.
    • In the extreme northern Sub-arctic region, early breeding behaviors are not evident until several weeks later, when daylight begins lengthening after the Winter Solstice.

A less obvious difference between populations is perhaps more significant: the timing of egg-laying, during the Sexual sub-phase of the Maturity stage (marked by pink shading). The distinction is whether eggs come while the photoperiod is increasing (marked by ↗↗ to ↗↗↗ arrows in the Photoperiod row at the top of the diagram), decreasing (marked by 2 arrows ↘↘ or 3 arrows ↗↗), or barely changing at all (marked by a single ↗ arrow).

    • Eagles in the northern latitudes (Sub-arctic, North Temperate, and Middle Temperate) lay eggs in late winter and spring when daylight is lengthening more and more rapidly.
    • Eagles in the Sub-tropics lay eggs in late fall or early winter as daylight is short or only slowly lengthening.

The early schedule of egg-laying for southern eagles has made some wonder whether photoperiodism plays any role at all in timing their Life History stages. Among scientists there is no real debate about this. While daily changes in the photoperiod in the south are much smaller than further north, studies have shown that birds throughout the Temperate zone and down into the Tropics are highly sensitive to small changes in photoperiod. (Dawson 2007) Like birds further north, birds who breed in the far south have endogenous circadian and circannual rhythms, they experience annual rhythms of gonadal recrudescence and regression as well as Molt, their bodily systems respond to the changing photoperiod, and their endogenous biological clocks are entrained to it. (Gwinner & Scheuerlein 1999) In fact, some birds in the Tropics (well south of the Bald Eagles’ range) can perceive and respond to a difference as small as 17 minutes between the shortest and the longest photoperiod of the year (Hau et al. 1998)—much shorter than the approximately 3.3 hours annual difference for eagles at the southern tip of Florida.

The differences in breeding schedules at different latitudes can be traced to adaptive (genetic) differences in the ways the neuroendocrine systems perceive and respond to the changing photoperiod. (MacDougall-Shackleton & Hahn 2007; Hahn & MacDougall-Shackleton 2008) Some studies suggest that the reproductive systems of populations at lower latitudes may not become completely inactive after the regular breeding season but remain minimally functional and can be reactivated quickly—that their Gonads do not regress completely, but if conditions are favorable they can recrudesce rapidly. (Immelmann 1972; Dawson et al. 2001; Dawson 2008)

Scientists have applied the term long-day breeders to species or populations who lay eggs during the increasingly longer photoperiods in spring and summer, which includes most birds. Experiments have shown that for these birds the initial predictive cue, the critical photoperiod that induces the onset of photosensitivity, occurs when daylight is decreasing after an extended period of long days—i.e. around the Autumnal Equinox. This is the pattern that most Bald Eagles follow. A few species of birds fall into the category of short-day breeders, those who lay eggs as daylight is decreasing or short in the fall or winter. For them, including many southern Bald Eagles, the onset of photosensitivity can occur in the summer while daylight is still long.

Researchers have found that these timing differences between long-day breeders and short-day breeders may be due to how their brains function. (Gwinner & Scheuerlin 1999; Sharp and Blache 2003; Sharp 2005)

    • In long-day breeders the minimal changes to photoperiod of the long days of summer render the neurons in the Hypothalamus insensitive to daylight. It takes the shift to larger changes in the photoperiod in autumn to “wake up” those neurons so they can begin responding to the changes and send signals to hormones in the Pituitary Gland.
    • In short-day breeders the neurons in the Hypothalamus are more active throughout the year and can function even with minimal input from the photoperiod. This means that their reproductive systems may be more readily controlled by their endogenous circannual systems and the neurons can begin to send signals downstream earlier.

(This may help explain events in 2019-2020 at the Southwest Florida eagles’ nest (Fort Myers, FL) where several weeks after losing their 37-day old chick the eagles produced a very late second clutch and raised those eaglets to fledge. A successful second brood after loss of eaglets (as opposed to eggs) from a first brood is a rare occurrence among Bald Eagles, and it may occur only in climates where their reproductive systems are adapted to make it possible.)

NON-PHOTOPERIODIC PROXIMATE FACTORS IN REPRODUCTION

Photoperiod is the only proximate factor that is consistent from day to day and year to year and thus the only one that a bird’s body can rely on as an initial predictive cue. But other proximate factors, which are often unpredictable or irregular, also can affect the timing of Reproduction (Wingfield et al. 2007). Such supplementary information can help fine-tune the timing of Reproduction in short-term situations. Researchers have found that the effect of some proximate exogenous cues on reproductive activities may be more significant in southern birds than in northern birds. (Dawson 2008; Hahn et al. 2009; Silverin et al 2008; Sturkie’s 6th: 504) Non-photoperiodic proximate factors include environmental cues and social cues.

Environmental cues tend to be variable and include climate, weather conditions, and food availability. Unpredictable environmental situations (storms, extreme heat or cold) can be disruptive and can delay or prevent reproductive activities, or even result in loss of a clutch, brood, or nest. Male breeders may be more affected by unfavorable environmental cues than females, since male sperm production may suffer if such conditions are persistent, even before females are ready to ovulate.

A particular region generally has a predictable temperature range, and it is tempting to wonder whether the significant differences in ambient temperatures across the latitudes of the Temperate Zone play a role in the timing of Reproduction. The theory is that the start of breeding is triggered to some degree by the onset of warm weather—earlier in the south and later in the north. But scientists have found little evidence that temperature plays a role as a predictive factor. (Cassone & Kumar 2015) Temperatures from day to day or week to week can make unexpected swings, and there is no way for a wild animal to foresee such swings far enough in advance to begin the long Reproductive process. Once the reproductive process has begun, though, a temporary stretch of unusually low temperatures can slow it down and abnormally high temperatures may speed it up.

Some regions have fairly regular seasons of rainfall, but as with temperature ranges these do not appear to have a predictive impact on the timing of breeding. Unusually low or high rainfall as well as extreme temperature swings can adversely affect the availability of food. Inadequate food in a particular year can stop a breeding season before it starts, and it can induce potential parents to delay or scuttle a breeding attempt, tax the health of breeding adults, or impair the growth and development of nestlings.

The clouds that accompany rainfall may reduce sunlight intensity, the amount of radiation—i.e., energy in the form of light and heat—that reaches the earth by scattering its radiation. Sunlight, of course, is coordinated with the photoperiod throughout the year, but its intensity is dependent on the time of year and time of day. At any given latitude, sunlight intensity is greatest and longest at the Summer Solstice, when it is at its maximum elevation of the year. It is greatest in the middle of the day when the sun is at its highest elevation. It is lowest in early morning and evening both because its lower angle in the sky increases the distance the radiation must travel to reach the earth’s surface, and because at those times its effect is distorted by atmospheric refraction. So if clouds are present at mid-day but build up late in the afternoon, they have less effect on the amount of sunlight intensity reaching the Earth.

Prolonged cloud cover over several days can lower the sun’s intensity, reducing the signals that a bird receives from the photoperiod. Such extended periods can happen almost anywhere, and it may affect birds in the short term, as for instance inducing females to delay ovulation or oviposition. (Dawson 2008) Some areas may experience a regular annual rainy season, suggesting the possibility that it affects the timing of Reproduction. For example, parts of Florida commonly experience a rainy season from June through September. (Daily details of weather features for a given location can be explored here.) Could this be a reliable trigger for gonadal recrudescence and the beginning of Reproduction activities? It is a question impossible to answer with what we know. But while it may be one factor in the timing of Reproduction, a general annual pattern of rain and cloud cover in a particular region can be variable from year to year, day to day, and hour to hour, and the effect on light intensity depends on the time of day that clouds are present. Such variabilities might make cloud cover unlikely to be as reliable a predictive factor every year as the changing photoperiod, although it could have played a role in long-term adaptation to a local climate. (Gwinner 2003; Sturkies 5th, 811; Ronneberg 2010)

Another pertinent point is that while Florida’s summer rains may have played some role in the adaptation of the Life History of the population of Bald Eagles there, a similar situation at comparable latitudes, such as Louisiana, South Carolina, Texas, and Georgia, is not evident. Eagles there have the same egg-laying schedule as Florida but not the same cloud cover pattern, so in those regions photoperiod is the only predictive cue to govern the Reproduction schedule.

Prolonged or persistent exogenous cues, such as climate change, can impact a species or a population over the long term by inducing permanent adaptations that enhance the likelihood of success, and thus become ultimate factors.

 Social cues include presence (or absence) of a suitable mate, appropriate interactions between male and female, competition for a mate or a territory, nest site availability, and human disturbance. Pair-bonding and nest-building stimulate hormonal secretions and bring a male and a female into hormonal and behavioral sync, which is crucial to successful breeding (see discussion in the article on Reproduction and Hormones).

Disruptions by intruders or humans are unpredictable and can delay or prevent reproductive activity. Female breeders generally are more affected by social cues such as effective (or inept) courtship behavior by a male. Female Bald Eagles are especially dependent on social cues in the final stages of the Maturity phase of egg production, to induce yolk deposition in the Ovary as well as ovulation. (Ball & Ketterson 2008) But both females and males are affected by other social factors such as intruders, loss of a nest, or human activities.

References: Photoperiod

References: Life History

REPRODUCTION AND HORMONES

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Reproduction in birds involves mostly the same or analogous hormones as in humans and other mammals, but the processes involved are affected by some significant distinctions between birds and humans, including:

    • Birds are oviparous (Latin ovi = egg + parus = producing), as opposed to viviparous (Latin vivus = living + parus = producing). They lay hard-shelled eggs with the barely developed embryos encased inside, along with all the nutrients the embryos will need while they develop.
    • A bird’s Reproduction season generally recurs every year (or two in some species). It is the central stage in a Life History that in many, if not most, species is repeated annually throughout a bird’s lifetime. In Bald Eagles Reproduction can occupy from 8 to 10 or 11 months of the year.
    • Because birds are wild, feathered, flying creatures, for most species the gonads are mature and active only during the Reproduction stage of the annual Life History. The “down” time for the reproductive system is an adaptation that prevents breeding at an inopportune time of year, and it enables the bird’s body to allocate its energy resources to Molt and post-breeding Movements. After each annual breeding the gonads regress to a minuscule and unproductive physiological state and must regrow every year to maturity (gonadal recrudescence) for the next breeding season.
    • The hormonal and other physiological (endogenous) changes that birds undergo and the behaviors they engage in during Reproduction are triggered by exogenous cues from the immediate environment, especially changes in the photoperiod. Physiology and behavior also can be affected by supplementary external factors such as extreme weather, social interactions, health of the pair, food availability, territorial disputes, and human disturbance. (Discussion of photoperiodism and how and when the Reproductive stage begins across North America is found here.)

Everyone knows about the so-called sex hormones produced by the Gonads (the Ovary, of which most female birds have only the one on the left, and the paired Testes) – Estrogens, Testosterone, Progesterone. Less familiar are the hormones that trigger those gonadal hormones, secreted from the Hypothalamus, a region of the brain, and from the two lobes of the Pituitary Gland that are below and connected to the Hypothalamus. (Go here for details on the Endocrine System and Hormones.) The Hypothalamus serves as the body’s bridge between the exogenous environment (light and other cues) and the endogenous physiological systems. It kickstarts the Reproductive process. The Pituitary Gland, often called the “master gland,” is in charge of sorting the multifarious signals it receives from the Hypothalamus and other organs and glands, as well as some exogenous cues, and determining what hormones downstream need to be activated and when. Among the hormones it produces are gonadotropins (= hormones that act on the gonads). The Posterior lobe of the Pituitary Gland does not produce hormones but receives and stores hormones from the Hypothalamus.

These structures and the hormones they produce are at the core of the Reproductive process, and researchers group them together as the Hypothalamo-Pituitary-Gonadal (HPG) Axis. Here is a schematic flowchart (not to scale) of what the HPG Axis looks like in birds (Orange boxes represent glands and organs, blue boxes represent hormones, green arrows represent positive effects, and red arrows represent negative effects. Actions of the hormones and the timing of their release are described in more detail below. Click on the image for an enlarged view):

HYPOTHALAMUS

GnRH = Gonadotropin-Releasing Hormone
VIP = Vasoactive Intestinal Peptide
MT = Mesotocin, stored in the Posterior Pituitary for later
release
AVT = Arginine Vasotocin, stored in the Posterior Pituitary
for later release
GnIH = Gonadotropin-Inhibiting Hormone

ANTERIOR PITUITARY GLAND

LH = Luteinizing Hormone, a gonadotropin
FSH = Follicle-Stimulating Hormone, a gonadotropin
PRL = Prolactin

GONADS (Testes and Ovary)

ESTR = Estrogens
TEST = Testosterone
PROG = Progesterone
INH = Inhibin

Other glands also produce hormones that play a role in Reproduction, including the Thyroid, which secretes T3 (= Triiodothyronine) and T4 (= Thyroxine), the Adrenal Cortex which secretes CORT (= Corticosterone), and the Adrenal Medulla which produces EPI (= Epinephrine) and NE (= Norepinephrine).

Biologists have established a broad outline of three phases of the Reproduction cycle in birds and the behaviors observed in each phase (adapted from Wingfield 1999; Bentley et al. 2007):

  • Preparation, including territory establishment, pair bonding, the beginning of gonadal recrudescence, sperm production, and yolk deposition in the ovarian follicles.
  • Maturity, which can be divided into two sub-phases:
    • Sexual Sub-phase, including nest finalizing, ovulation and fertilization, and oviposition
    • Parental Sub-phase, including incubating eggs and caring for chicks
  • Termination, including juvenile fledges and dispersal, and adult movements

Here is how these phases fit into the full annual Life History of Bald Eagles (click on the image for an enlarged view):

The duration of each phase can depend on latitude, stability of the pair, territorial challenges, weather, human disturbance, etc. Details and sometimes the sequence of events can differ from one avian species to another, as well as among individuals within a species. Hormones are released in sequence as these phases progress. Here is how the Reproduction phases play out for Bald Eagles:

PREPARATION/DEVELOPMENT PHASE

This is a critical phase of Reproduction, and it can extend from 8-16 weeks for Bald Eagles. Gonadal recrudescence is a slow process, and to maintain its progress male and female Bald Eagles must engage in preparatory behaviors of claiming their territory and pair bonding, including flying and roosting together, sharing food, vocalizing, and nest-building. The social interactions between the pair are important factors in stimulating hormonal secretions and establishing the delicate balance between them that enables their reproductive systems to develop synchronously, eggs to be fertilized, and parental care to be mutual and effective.

The length of this stage depends on whether the bond is already established and an existing nest is available. New pairs may begin with one adult from a previous season who is courted by one or more potential mates. Sometimes an entirely new male and female take over a nest. A new nest can be built within a week or two if necessary, but hormonal synchrony still must develop over time.

These bonding behaviors reinforce the daily photoperiod changes that photoreceptors in the Hypothalamus perceive, telling the pair that it is the time of year in their Life History to enter the Preparation phase of Reproduction. The Hypothalamus in each bird then activates a gradual and steady increase of hormonal secretions, as this flowchart shows. (Faded arrows indicate hormonal actions that are absent or minimal during this phase. Click on the image for an enlarged view.)

Gonadal recrudescence
GnRH begins the hormonal cascade by triggering the release of the gonadotropins LH and FSH, and these respond by stimulating production of the gonadal hormones ESTR, TEST, and PROG and their release to the oviduct and the male’s sperm duct (the vas deferens). Together, these gradual secretions begin the long process of gonadal recrudescence and stimulate territory establishment and pair-bonding activities.

Copulation becomes more frequent as the phase progresses, which brings about a rise in circulating levels of Thyroid hormones T3 and T4 (not depicted in the flowchart), which play a role in spurring gonadal growth and the production of sperm and ova.

The Hypothalamus secretes the hormones MT and AVT to the Posterior Pituitary to be stored for later release.

Spermatogenesis, yolk deposition, and ovum development
After a few weeks, the Gonads have reached a certain state of development and LH and FSH stimulate the Testes to begin producing sperm (spermatogenesis), and an ovum to begin developing in the Ovary. A yolk (with lipids and proteins for the future embryo) begins to form in the first ovarian follicle in line (F1). LH secretion in the male reaches a peak near the end of the Preparation phase, and shortly thereafter in the female.

Brood patch
GnRH secretion peaks toward the end of the Preparation phase, while secretion of VIP begins to increase, stimulating production of PRL, which along with ESTR and PROG initiates development of brood patches.

MATURITY/NESTING PHASE:   SEXUAL SUB-PHASE

This is the briefest but most intense period of the breeding cycle. Reproductive hormones become fully activated. Over the course of only 3-4 weeks, parents finalize the nest, copulation increases dramatically, brood patches near completion, eggs are ovulated and fertilized, eggshell is laid down in the uterus, embryonic development accelerates, and eggs are laid (oviposition). Click on the image for an enlarged view.

Gonadal maturation
GnRH, LH, FSH, and gonadal hormones ramp up significantly and rapidly, bringing the Gonads to maturity.

FSH peaks about halfway through this sub-phase. It encourages nest completion, induces increased spermatogenesis and final yolk deposition in the ovarian follicle (F1). FSH remains high until all eggs are laid.

LH also helps stimulate increased spermatogenesis. It rises a little more slowly in the female and boosts ESTR and TEST levels in the Ovary, stimulating yolk deposition in the ovarian follicle (F1) and development of the ovum.

Brood patch
VIP secretion increases, further stimulating release of PRL, which rises dramatically as ovulation approaches, working with ESTR and PROG to finalize development of brood patches. De-feathering is nearly complete by the time ovulation occurs, although full vascularization is reached later during the incubation period.

Ovulation, fertilization, and embryonic development
TEST and ESTR peak a few hours before ovulation, then decline gradually to ovulation. Both contribute to inducing sperm production and maturation of F1 in the Ovary, and ESTR prompts production of albumen in the oviduct. ESTR and PROG may play a role in determining the gender of the embryo. ESTR, along with PTH (= Parathyroid Hormone) from the Parathyroid Gland (not depicted in the flowchart), is instrumental in increasing the concentration of calcium in the Uterus for eggshell production.

LH inhibits production of INH until ova are approaching maturity, when INH assumes an essential role in ovulation. INH complements the action of FSH so that only one egg ovulates at a time. FSH induces maturation of F1 while stimulating INH, which inhibits development of the less developed follicles (F2, F3, etc.). After F1 is ovulated it is fertilized quickly, and the next follicle in line becomes the target of FSH while INH shifts its inhibitory action to the follicles behind it. This ebb and flow repeats for each ovulation. INH then helps suppress production of GnRH, LH, and FSH to bring egg production and ovulation to an end.

PROG increases gradually and spikes just before ovulation. Immediately after an egg is ovulated, PROG stimulates stored sperm to move up the oviduct for fertilization.

PROG and ESTR send positive feedback to GnRH, LH, and FSH. LH surges just before ovulation and remains high until all eggs are laid, then declines quickly.

Thyroid hormones (not depicted in the flowchart) are critical in the female, who must produce enough to supply her own needs and those of the developing embryo. If her levels are insufficient, she may not be able to lay eggs, or the embryos may not develop properly. (McNabb & Darras 2015, 541) T4, which also contributes to gonadal recrudescence, peaks early during the Sexual Sub-phase and then is suppressed by gonadal hormones.

Oviposition
MT, with assistance from ESTR and PROG, dispatches AVT to the Uterus to induce uterine contractions. PROG begins to decrease slowly after ovulation. The ovulation and oviposition cycle of Bald Eagles repeats every 3 or 4 days until all eggs of the clutch have been laid.

Gonadal regression
GnIH (the inhibiting hormone) secretion begins after ovulation, helping suppress LH and FSH and leading to a decrease in gonadal hormones and the beginning of gonadal regression.

The pace of PRL secretion and its amplitude pick up when ovulation comes to an end. It is further stimulated by VIP and AVT, inhibiting LH and FSH and further growth of the Gonads after a clutch is complete.

GnRH begins to decline slowly after the last oviposition, while LH and FSH decrease more rapidly, contributing to gonadal regression.

MATURITY/NESTING PHASE:    PARENTAL SUB-PHASE

The decline in reproductive hormones results in less intense sexual interaction between male and female and encourages “broodiness” as researchers often describe the parents’ demeanor during this period of unflagging attention to eggs and then chicks. The Parental sub-phase begins with the long period of incubation of about 36-42 days, depending on the number of eggs in the clutch. This period is known by nest cam viewers as the “slow” season when there is little to watch other than eagles taking turns incubating and occasionally bringing sticks and other materials to shore up the nest. Sharp-eyed observers may begin to see stray feathers in the nest, as Molt begins about halfway through incubation. When the eggs hatch, demands on the parents intensify dramatically as they tirelessly feed and protect the growing eaglets from weather and predators until they fledge after about 10-13 weeks. Click on the image for an enlarged view.

Gonadal regression
GnRH continues its slow decline through incubation, as GnIH continues to rise, inhibiting secretion of LH and FSH, leading to a decrease in gonadal hormones and furthering gonadal regression.

PRL is still stimulated by VIP and AVT and continues to suppress GnRH and gonadal hormones.

Incubation and Molt
PRL is stimulated also by the presence of eggs, inducing incubation behavior. It may also help activate Molt.

ACTH (= Adrenocorticotropic Hormones) from the Anterior Pituitary (not depicted in the flowchart) stimulates secretion of CORT (= Corticosterone) from the Adrenal Cortex (not depicted in the flowchart) peaks, helping induce incubation behavior and depress gonadal hormones. ACTH secretion increases about halfway through incubation and contributes to the suppression of GnRH and gonadal hormones. It helps increase circulating Thyroid hormones T3 and T4 (not depicted in the flowchart) which begin to stimulate feather growth and Molt in the incubating parents.

Brooding eaglets
In Bald Eagles and many other altricial species, PRL remains high through the Parental sub-phase while other hormones begin to decline, maintaining parental behavior and further dampening sexual interactions. PRL tapers off only slightly and may help stimulate Molt.

TERMINATION PHASE

After the juveniles fledge, both eagle parents generally continue to provide food as their young learn flying skills, watch the adults closely for thievery opportunities, and probably pick up pointers about where and how to find food. The phase can be as short as a week (depending, for instance, on whether the parents disperse quickly to areas of better food availability, as they do in the Pacific Northwest), or as long as 7-8 weeks. Gonadal regression is nearly complete. An increase in CORT in the parents as the fledglings disperse helps reduce reproductive hormones to their base levels. Fewer demands on the energies of the adults allow Molt to accelerate (the juveniles retain their first contour feathers for a year), although it can be interrupted during periods of Movement. T4 continues to stimulate feather growth.

TIMING IS EVERYTHING

Behaviors, hormonal secretions, and physiological changes work together, each affecting and reinforcing the others during every phase of the Reproduction stage. Hormones induce endogenous changes such as gonadal recrudescence, as well as activities such as copulation and nest-building. These behaviors in their turn stimulate further hormonal and biological changes in the mate, and so on in a continuous feedback loop. This circularity is key to establishing behavioral and physiological synchrony between female and male, especially during the Preparation phase. A relatively short Preparation phase that might result from disruptive events like severe weather or replacement of a mate may hamper the development of synchrony between the pair, which could result in unequal timing of ovum and sperm production and hence fertilization, or disharmony in parental activities.

The timing and pace of the secretion of reproductive hormones among most avian species are similar, within the bounds of each species’ Life History. Using data and information from many sources, I have created an amalgam schematic diagram of the characteristic reproductive hormone secretions within the framework of a typical 40-week Reproduction stage of a pair of Bald Eagles. (This diagram is not a representation of actual data but depicts an approximation of the rise and fall of each hormone in relation to the others and to events during Reproduction. Some activities are likely to occur within the same time frame, e.g. nest building and copulation. Blue lines represent male secretions, orange lines represent female secretions. Click on the image for an enlarged view.)

Several things to note:

    • Secretions of GnRH, LH, FSH, and TEST occur earlier in males (blue lines) than those of females (orange lines), and the Testes develop more rapidly than the Ovary. This means that spermatogenesis occurs much earlier than ovum maturation, which is a more complicated process that does not reach fulfillment until just before ovulation. All the copulation in the world will not result in fertilization until an egg has been ovulated, but it does strengthen the bond between male and female. It also enables a male to implant perhaps millions of sperm in her oviduct for storage. (But it is uncertain how long sperm remain viable after insemination, and by the time an ovum is ready, recent sperm are more vigorous and better able to compete for fertilization rights than older sperm.) The earlier start of males in the Preparation phase is often noted by Bald Eagle nest cam viewers who sometimes see breeding males appear and begin nest-tending sooner than females do.
    • Conversely, female secretion of GnRH, LH, and FSH usually remains more elevated later into the Parental sub-phase than in males. If a clutch of eggs (or in rare instances, of chicks) is lost, this elevated level keeps the female at a sufficient state of readiness to recycle her secretions to produce a new clutch of eggs.
    • PRL rises dramatically in males and females just before eggs are ovulated, while the reproductive hormones are beginning to decline and gonads are regressing. PRL generally remains at a high level through the parental care period.
    • The timing difference between the secretion of PRL and the reproductive hormones is clear in this diagram, with the rise of PRL lagging the others. This is crucial to breeding success. If the biology and behavior of a pair are in step, ovulation and fertilization occur before PRL induces both female and male to share incubation duties. But this harmony is disrupted if one member of a breeding pair is lost and a new adult attempts to establish a bond with the remaining breeder. If this occurs early in the Preparation phase there still may be time for them to achieve synchrony, but it becomes much less likely as the Maturity phase approaches. This is especially so if it is the male that is lost and a new one appears. His gonads may recrudesce quickly and begin producing sperm, but he may have missed out on the pair-bonding activities that are crucial to a pair’s synchrony. Further, the disruption may have caused her reproductive system to shut down, preventing ovulation. Even if she is able to ovulate, and his sperm succeed in fertilizing her eggs, his GnRH, LH, and FSL may still be rising while hers are declining, causing him to persist in sexual behavior while her focus will shift to incubating as her PRL ramps up. She will bear all the burden of caring for the eggs, and probably will need to leave the nest to find her own food. This disconnect is likely to continue until they both abandon the breeding attempt.
    • CORT becomes elevated near the time of oviposition, helping depress secretion of reproductive hormones and encourage incubation. It then tapers off but rises again in response to PRL, to aid in preparing the parents for the intense demands of caring for new nestlings.

Any phase of Reproduction can be interrupted by events such as extreme weather, territorial challenges by other eagles, owls, larger hawks, or ospreys, loss of a mate, a nest, or the food supply, or human disturbances. The Hypothalamus responds quickly to serious unexpected disturbances by secreting CRH (= Corticotropin-Releasing Hormone) to the Anterior Pituitary, which sends ACTH to the Adrenal Glands. The Adrenal Cortex immediately releases CORT and the Adrenal Medulla secretes EPI and NE. These hormones quickly suppress secretion of reproductive hormones and induce appropriate responses by the eagles, including suspending breeding behaviors, taking shelter, challenging intruders, or vacating the area. Ovulation may be delayed if the normal LH surge just preceding it does not happen, or the cessation of secretion of AVT to the Uterus may postpone uterine contractions. The reproductive hormones may restart if adverse conditions are not prolonged, and the bonded pair could resume hormonal and behavioral development. If difficult events are prolonged, they may hamper or suppress the cycle of reproductive hormones completely, causing the breeding attempt to fail.

References: Endocrine System – Hormones

LIFE HISTORY or ANNUAL CYCLE

© elfruler 2023

Birds—like all wild animals—must maintain a balance between the benefits and costs of the activities that enable them to survive and thrive as species and as individuals. To maintain their evolutionary fitness, almost all species undergo a repeated cycle of activities and physiological changes, a genetically hard-wired program adapted over eons of time that is referred to as a species’ Life History. A species’ Life-History traits are defined in the Handbook of Bird Biology HBB 3rd ed. (2016) as “the fundamental traits that directly influence an individual bird’s survival and reproduction.” These traits include lifespan, size, plumage, molting pattern, diet, age of sexual maturity, number of offspring each breeding cycle, pace of embryo and nestling development, the roles of females and males during incubation and brooding, territorial habitat, movements, and rate of survivorship.

Taken together, these traits engender Life-History strategies that vary widely among the over 11,000 bird species on the planet. These strategies play out in a repeated series of stages that for most species mirrors the Earth’s annual cycle of changing seasons caused by its revolution around the sun every 365.24 days and the ~23.5° tilt of its rotational axis.

    • Reproduction is the central Life-History stage of territory establishment, nesting, laying eggs, and raising chicks, obviously essential to a species’ survival. Reproduction controls the timing of the other Life-History stages.
    • Molt is the shedding of old and regrowth of new feathers throughout the body. It is vital to all birds because although feathers have inherently strong structures, they become frayed and damaged by wear and tear and must be replaced regularly for a bird to be able to survive.
    • Migration is a period after breeding when birds settle in areas with adequate food resources and favorable climatic conditions, often travelling long distances, before the next season of Reproduction begins.

Reproduction and Molt are common to all birds. Migration is widespread throughout the avian class but it is not essential for all birds. A number of species, including many Bald Eagles, are able to find sufficient food in or near their breeding territory and some have adapted their diets and physiologies for different seasons. To reflect the fact that not all Bald Eagles migrate, I use the term Movement for this stage in Bald Eagles (following Buehler 2022, Wheat et al. 2017).

The timing of Reproduction in the annual calendar depends on the availability of optimal food resources for both the needs of the young as they develop and of the parents whose energies during the breeding season are especially taxed. Ornithologists view food availability as the ultimate factor in the adaptation of the timing of the Annual Cycle: It is the root factor determining the evolution of a species’ Life-History traits and strategies.

Each Life-History stage entails high energy expenditures, so the respective peaks of Reproduction, Molt, and Migration cannot occur simultaneously, although there can be overlap between the winding down of one stage and the beginning of the next (Wingfield 2005). While some species routinely repeat the Reproduction stage with multiple clutches of eggs, among Bald Eagles a second clutch occurs only after loss of one clutch or brood. Some species undergo more than one Molt annually, but Bald Eagles have only one annual Molt. Life History strategies may be modified in the short term by unusual or unexpected environmental factors such as extreme weather, scarcity of food resources, or territorial disputes. The Life-History traits also may be expressed differently by individuals of a species, as well as by a regional population of a species (e.g. Bald Eagles in the southeastern U.S. or in Alaska). (Go here for detailed discussion of timing differences among populations of Bald Eagles in relation to Photoperiod.) Longer-term or permanent changes to Life Histories can occur in response to the effects of climate change, habitat loss, and human disturbance.

Here is a schematic diagram of the Life-History stages of Bald Eagles, mapped onto 52 weeks of the year (divided into 4-week units that do not necessarily correspond with January through December). Timings here are approximate and represent a rough average of observations made at eagle nest sites (including those on cam), as well as information from published literature. Click on the image for an enlarged view.

REPRODUCTION

As the above chart shows, the Life-History stage of Reproduction of Bald Eagles is quite long, extending well more than half of the year, and in some regions or for some eagle pairs it can occupy 10-11 months. Reproduction, like each Life-History stage, is characterized both by major physiological changes and by noticeable behavioral changes. The most significant physiological change during Reproduction is in the Ovary and Testes, which after Reproduction each year shrink to a minimal size and cease to function. The following year they must redevelop back to a state of maturity before a new Reproduction effort can occur. The process of regrowth is referred to as gonadal recrudescence, and the contraction of the Gonads is referred to as gonadal regression. The gonadal and other physiological changes and the behavioral changes are both controlled by changing hormonal secretions. (The Avian Endocrine System in general is described in detail here, and the complex endocrine processes associated specifically with Reproduction are explored here.)

Reproduction proceeds through several sub-stages or, as I will refer to them, phases:

    • The Preparation phase. Breeding eagles begin a transition from the quiet period after Molt and Movement. They establish or reclaim a territory, develop or strengthen a pair bond, and begin working on their nest. Hormones from the Pineal Gland, the Hypothalamus region of the brain, the Pituitary Gland, and the Gonads themselves stimulate these preparatory activities and induce the Gonads to begin their recrudescence. The secretions slowly increase through the Preparation phase, and they stimulate increasing incidents of copulation, sperm production in the males, and the beginning of yolk formation in the liver and deposition in the most mature ovarian follicle in advance of ovulation. For Bald Eagles, the Preparation phase can last from 8-16 weeks (average about 12 weeks).
    • The Maturity phase. This is the most intensive of the phases, when the eagles engage in the full range of reproductive activities. Two sub-phases are evident during the Maturity phase:
      • The Sexual sub-phase. Copulation accelerates, nest preparation is finalized, yolk formation and deposition in the Ovary are completed, and egg production (ovulation) and laying (oviposition) occur. Hormonal secretions reach a peak during this sub-phase, inducing production of gametes by both male and female, development of brood patches, fertilization of eggs, shell deposition around the developing embryo growth, and oviposition. For Bald Eagles this sub-phase can occupy 3-4 weeks.
      • The Parental sub-phase. This behavior sets in after a clutch of eggs is complete. Incubation begins, and when eggs finally hatch, the parents transition into a new demanding role of brooding and providing food and protection as the chicks grow. Most reproductive hormone levels decline markedly during the Parental sub-phase. The adults gradually spend less time in the nest (often in self-defense as the eaglets become aggressive in snatching food that arrives). For Bald Eagles incubation and chick-rearing can take from 15-18 weeks (average about 17 weeks).
    • The Termination phase. The parents transition into Molt, which begins about halfway through the incubation period for Bald Eagles. Hormonal secretions drop toward their base level, causing the Ovary and Testes to regress. This is important because it renders reproduction impossible at a time of year when food resources for growing chicks are likely to be less plentiful. It also frees up metabolic energies for Molt and Movement. The eaglets fledge, practice flying, obtain or steal food from their parents and sometimes procure it for themselves (especially if carrion is available), and finally disperse away from their natal territory. The parents may stay in their territory but will rarely be seen in the nest, or they may travel further away in search of food (see below). The time from fledging to dispersal and the Movement stage for adult Bald Eagles can be 1-8 weeks (average about 4 weeks).

Bald Eagles lay only 1 clutch of eggs each year unless those eggs (or rarely, chicks) are lost early enough to make a second clutch possible. If the gonads have not regressed completely and can be induced by renewed secretion (sometimes called “recycling”) of reproductive hormones to enable ovulation and fertilization, the Sexual and Parental sub-phases can be repeated. A successful second clutch also is contingent on continued availability of adequate food to meet the needs of both the chicks and the parents.

These phases and sub-phases of Reproduction can be added to the schematic diagram of Bald Eagles’ Life-History stages (above). This diagram allows for the likelihood of at least 2 eggs in the clutch (adapted from Wingfield 1999, Bentley et al. 2007). Click on the image for an enlarged view.

MOLT

Like Reproduction, Molt exacts intense metabolic costs, requiring careful coordination of neurological, hormonal, and physiological processes as well as sufficient nutrients for the generation and growth of new feathers. Because of these heavy demands, Molt cannot occur during the first part of the intense Maturity phase of Reproduction, but it may begin after the gonads have begun to regress. Molt proceeds slowly in breeding adults until fledglings have left the nest, after which it progresses more quickly; for non-breeders, molt is steady from the start (Jacobs and Wingfield 2000, 43). It may be suspended when demands on breeders escalate during parental care of hungry chicks, during the Movement stage, or when other situations divert metabolic resources, like food shortages, severe weather, injury, or territorial challenges. Molt proceeds from the top down: first the head and neck (whose feathers are short and replaced relatively quickly), then body and wings (except the wing flight feathers, which begin to molt early in the cycle, along with the head and neck feathers), and finally the tail.

Various molting strategies have evolved to serve the needs of species with different Life Histories, feather types and arrangements, courtship and hunting behaviors, diets, habitats, and so forth. Probably the best-known strategy is found among songbirds, which molt in the spring from a drab “winter” plumage to a more colorful spring “breeding” plumage (especially the males), and then again in late fall back to the winter plumage. Another strategy is followed by many waterfowl who undergo a “synchronous wing molt,” during which all of the wing flight feathers are replaced at the same time, making the birds flightless for that period and requiring them to find hiding places so that they are not visible to predators.

The molting strategy of Bald Eagles begins with the fact that they have only 1 Molt each year (no “breeding plumage” or “winter plumage”). It begins about halfway through the incubation period and continues well into the Movement stage. It is a partial molt each year. Unlike waterfowl, Bald Eagles cannot hide or feed themselves if they lose all their feathers at once and are unable to fly. The Molt stage is not long enough for them to replace all of their wing flight feathers (the remiges) one by one during a single year. The longest wing feathers of adults are from about 36-49 cm long (Feather Atlas) and it can take up to 75 days for one of them to grow in depending on such factors as body mass, latitude, and environmental conditions. So eagles replace only some of the primaries and secondaries each year following a strategy called stepwise or wave molt. This involves replacement of 4-7 of the 10 primaries and 3-11 of the 16 secondaries on each wing each year, in patterns that minimize the number of feathers molting at a time, their location on the wing, and the timing of any gaps where a feather has dropped out and a new one is growing in. Thus it may take 3-4 years for all of the remiges to be replaced.

The timing of Molt and the number of remiges replaced each year vary by latitude. Southern eagles have a longer timeframe each year during which they can replace their feathers, so they may molt all or most of their primaries and secondaries. Eagles further north with a shorter Molt season may molt half or fewer of their remiges each year (Clark 2001), and possibly only 6-7 of their tail feathers (rectrices). The annual average across North America is about 4-5 primaries (of 10) on each wing and 6-8 secondaries (of 16) on each wing. Females, which are larger than males and hence have slightly longer feathers, may begin Molt sooner, and if they are breeders they may conserve their energy for Molt during incubation and brooding by letting males do more of the food procurement. Overall, it may take females more years to replace all of their flight feathers than males.

Juveniles keep their first contour feathers for a year and have their first Molt beginning in the first spring after they hatch. The sequential molt of remiges that all eagles undergo is easiest to follow in growing eagles. That is because a juvenile’s remiges are longer by a few millimeters than the adult feathers that replace them. As they are molted sequentially in their 2nd, 3rd, and 4th years, the new feathers are visibly shorter than the juvenal remiges that have not yet molted, which gives the trailing edges of the wings, especially in the secondaries, a jagged appearance in the first 2-4 years. This is often visible from the ground and notably on hawk watches where observers attempt to age the younger adults as they fly over. Because the Molt of juveniles and sub-adult eagles follows a predictable pattern of replacement from one year to the nest for a particular region, careful observation of the number and location longer feathers, especially the secondaries, can be a more reliable indicator of a young eagle’s age than the gradual change of the head and tail feathers from dark brown to white. With an eagle held in the hand, older flight feathers can be recognized because they are frayed and have faded from dark brown to lighter brown, which may aid aging even adult eagles.

Several hormones affect the timing of Molt. A decline in the secretion of Testosterone and Estrogens in the gonads as the Reproduction stage winds down allows an increase in thyroid hormones, which help induce the beginning of molt and enable feathers to grow. Although a causal association has not been established, Prolactin peaks at about the same time that gonadal regression and Molt begin and remains relatively high through at least the first part of molt.  (See details about hormones here.)

References: Feathers & Molt

MOVEMENT

After the Reproduction stage of Life History is concluded and Molt is nearly complete, changes in the season not only expose birds to increasingly challenging weather, but also may affect the availability of food, so many birds migrate to areas where the climate is more friendly and food is readily abundant. Not all species, or even populations or individuals within a species, migrate. Ornithologists have used the term “complete migrants” for species that move away from their breeding range, sometimes traveling extraordinarily long distances.

Among North American diurnal raptors, Broad-winged Hawks, Swainson’s Hawks, Rough-legged Hawks, Ospreys, Swallow-tailed Kites, Mississippi Kites, and Turkey Vultures are complete migrants. But most raptors, including Bald Eagles, are not complete migrants, but are sometimes referred to as partial migrants, meaning species for whom some individuals or populations make long-distance journeys while some individuals or populations do not. For partial migrants, Migration is not a critical Life-History stage like Reproduction and Molt, because they have adapted to be able to survive even under challenging environmental conditions. As stated above, to reflect the fact that not all Bald Eagles migrate, I have adopted the term Movement here.

For most passerines and waterfowl in North America, the Reproduction season is in spring and summer. But the peak of the breeding season of Bald Eagles stretches from late fall through winter into early spring. Eagles lay eggs, incubate, and brood chicks from November (in the southern extent of their range) through April (in the far northern regions).  (See this page for details on Reproduction and Hormones.) Spring-summer breeders head south away from their breeding territories after the conclusion of their breeding season, but Bald Eagles who migrate tend to head north. Conversely, in the spring songbirds, geese, and other complete migrants who have over-wintered in warmer climates in the southern U.S., Central America, and parts of South America fly north back to their breeding territories. But Bald Eagles who have traveled north away from their nesting territories head back south to begin the Reproduction season. So in this sense, the movements of Bald Eagles are consistent with the general cliché that in the fall “birds fly south for the winter,” but at that time they are moving toward their breeding territories, not away from them.

The movements of most species of raptors are extrapolated from a limited amount of data. In relation to the size of their populations, only a tiny proportion of raptors have been tracked by tagging or monitored by ground observers reliably enough to confirm the movements of a particular individual. Many studies have reported such information about Bald Eagles, most of them younger birds that have been tagged in their first year and followed through subsequent years if the bands have been seen or recovered. The increase in the use of electronic geotrackers has provided much more detailed information, but as with earlier methods, such projects necessarily focus on a limited number of birds in a small region. In most cases, we simply don’t know where a particular Bald Eagle travels.

The exponential growth of the use of eBird, a product of the Cornell Lab of Ornithology, is a great boon to researchers. The data provided by thousands of observers who pinpoint a bird’s location, date, and time can yield extraordinary snapshots of the distribution of a species across its full range. But eBird is not a systematic data-gathering tool. It shows a random sampling of sightings made by viewers who happen to be eBirders and happen to be observing and reporting on a particular day. It is not a tracking tool that shows a particular bird’s movements, nor does it differentiate ages or sexes of most of the birds sighted.

eBird data is incorporated in the sightings maps on the Cornell Lab’s popular website All About Birds. The map of Bald Eagle sightings illustrates the full range of the species throughout the year. It clearly shows that all Bald Eagles remain in North America and blanket most of the continent from January through December. Members of the Cornell Lab (which is free) can access eBird’s Status and Trends section, which provides visualizations of seasonal abundance and trends correlated with the species’ Life History across their range. The animations suggest some movement north from March through August as the breeding season comes to an end, and movement south from September through January. But they also show that Bald Eagles populate most of North America throughout the year.

A more precise source of information about Bald Eagles’ movements is the Bird Migration Explorer tool developed by the National Audubon Society. It uses tagging and tracking studies and eBird sightings to produce animated maps showing the annual journeys of raptors and over 450 other species. Here is the map for Bald Eagles. Each dot represents an individual eagle, and clearly the data is sparse and widely scattered, illustrating how small the amount of available detailed information is. (Use the scroll bar at the bottom of the map to control the playback of movements through the year. Trailing shadows show the direction an eagle is moving. You can click, hold, and drag the scroll bar to move at your own pace.)

The animations illustrate points made above, that Bald Eagles are partial migrants, that many move north in the spring and south in the fall, and that all of them remain in the species’ range of North America. They also show that some eagles move in directions other than north and south.

    • In January most Bald Eagles are not moving much (most of the breeders are settled into their nesting territories), but some movement begins in late January and accelerates in late February.
    • By about the middle of March, as spring begins and the Reproduction season is winding down, more eagles are moving, largely northerly, but some head west (from the east and around the Great Lakes), east (northern and mid-Canadian provinces), and even south (Midwest, Pacific Northwest, along the Rockies, and the eastern U.S.).
    • By the end of May many eagles have settled again into local areas for the non-breeding season, although some (mainly juveniles and sub-adults) are still on the move intermittently.
    • In late September and the start of fall, extensive traveling begins again, mostly in a southerly direction, with some exceptions.

The animations on the Bird Migration Explorer website also indicate that many Bald Eagles move very little throughout the year. At fall hawk watches of migrating raptors such as I regularly attend, Bald Eagles are almost always among the fewest individuals to fly over, usually surpassed by Turkey Vultures, accipiter hawks, and many buteo hawks. (I also have to remind myself that when I see a Bald Eagle flying south in autumn, it is moving toward its Reproduction area, not away from it.). The Hawk Migration Association of North America (HMANA) maintains a detailed database of species reported at hawk counts all over North America, and it generally shows that Bald Eagles are among the fewest individuals of a species seen on a given day.

The relatively few published tracking studies of Bald Eagles reveal a variety of movement strategies during the Movement stage in the spring in particular, but also in the fall. Many of the studies provide information on the tracked birds’ ages, social status (mated or not), and habitats. The data suggests that the direction, distance, destination, and timing, of Movement is primarily determined by location of food resources, and also can be significantly affected by an eagle’s age, whether it is a breeder or is unmated (or a “floater”), and by factors such as latitude, topography (rivers, mountains, lakes, cliffs, plains, coasts), and weather events or other disruptions. The strategies include:

    • Local movements with little or limited travel from an area;
    • Nomadic movements with unpredictable directions, distances, destinations, and durations;
    • Migratory movements from one discrete area to another.

These strategies tend to correlate with the age and social status (mated or not) of an eagle:

    • Breeding adults. Even before their eaglets fledge, breeding eagles spend less time in the nest and may be absent for several hours at a time, although they usually don’t go far. After fledge, unfettered by nesting responsibilities, the parents are entirely free to roam more extensively. Some breeders are year-round residents of their local nesting territory as long as food sources remain ample. Other breeders relocate to areas where food may be more accessible and abundant, either within their own territory or in another breeding territory, an activity sometimes called dispersal. If they disperse from the immediate nest area, they, along with non-breeding adults, may do so before the fledglings depart. Most breeders undertake some movement away from the nesting area, even if only a few miles and for only a few weeks, before returning to prepare for a new Reproduction season. Breeding adults usually remain faithful to their own breeding territory year after year and may defend it against takeovers even during the non-breeding season.
    • Non-breeding adults. These floaters tend to be nomadic until they can claim a mate and establish a territory. Some of them remain in a local area for much of the year, if food is readily available. Some of these may hang out near a mated pair’s territory in hope of a takeover if the opportunity arises.
    • Juveniles (in their first 12 months) and sub-adults. Most sub-adults, like floaters, are wide travelers until they reach maturity and begin to breed, either in nomadic or migratory fashion. Juveniles often are more adventurous, more nomadic than adults and sub-adults and usually moving farther, sometimes dispersing from their natal territory to sites several hundred miles away. In the spring juveniles especially, and often sub-adults, generally depart an area after breeders have left. They tend to return eventually to the neighborhood of their natal nests in the fall, usually before the breeders or floaters have come back. When they reach breeding age they establish their own nesting territory in an unclaimed territory in the same area, or they may attempt to take over a nest or territory and replace a member of a mated pair.

During the winter while breeding adults are tending their eggs and chicks, floater adults, sub-adults, and juveniles usually find a roosting spot near a good food source, where eagles of all ages gather into convocations, especially along waters that remain open through freezing temperatures (like along a short stretch of the Chilkat River in southeast Alaska where salmon spawn, or the turbulent waters downstream of some of the locks and dams on the Mississippi River). Eagles in convocations spend many hours loafing during the day, but at feeding times they are in constant motion, swooping over the waters in the hunt for prey and often performing in-flight acrobatics in stealing attempts.

Migrating Bald Eagles never move at night, always during daylight hours when thermals and winds are active and visibility is good. They do not travel long distances in groups, generally solo or with one or two other eagles (a mate, floaters, or younger eagles), and often mixed in with hawks. Adult Bald Eagles and older sub-adults (third- or fourth-year) can move more quickly than juveniles, at least partly because juvenal flight feathers are longer and better at maintaining lift while soaring but less effective at maintaining thrust.

When the breeding season winds down, sex hormonal secretions decline rapidly and the gonads regress. Thyroid or Adrenal hormones increase during the Termination phase of Reproduction, and these may help induce Migration. Go here for details about hormones. Experiments on captive migratory birds have shown that they may exhibit nighttime restlessness, agitated behaviors in the period that would precede Migration. This has been interpreted as an urge to begin migration (sometimes referred to as Zugunruhe [German, Zug = journey, Unruhe = restlessness]). It may be related to lower amplitudes of Melatonin circulating in the blood, which reduces sleepiness and may enable migrants to adapt better to different time zones (Gwinner 1996; Cassone and Westneat 2012; Cassone 2014). But it has not been demonstrated conclusively that non-migratory or daytime migrating birds like raptors experience this restlessness.

References: Movement and Migration

References: Life History

THE AVIAN ENDOCRINE SYSTEM

© elfruler 2023

The Endocrine system is a complex network of organs, cells and tissues, chemicals, and processes that regulate and coordinate most of the body’s functions and keep it in a state of metabolic and behavioral equilibrium (homeostasis). It directs routine processes like breeding, molt, and growth, and also responds to unpredictable situations like bad weather, threats, or injury. The Endocrine glands include the Pineal Gland, Hypothalamus, Pituitary Gland, Adrenal Glands, Thyroids, Parathyroids, Ultimobranchial Glands, Pancreas, and Thymus. Other organs also play a role in the endocrine system, including the Kidneys, Gonads (Ovary and Testes), and Heart. (References: Endocrine System – Hormones)

WHAT ARE HORMONES AND HOW DO THEY WORK?

The glands and organs of the Endocrine system produce hormones, chemical substances that are secreted into the body for a specific purpose. A particular hormone affects only tissues that have receptor cells for that hormone, proteins that are capable of binding with and undergoing a chemical reaction to it. So, for instance, the Insulin secreted by the Pancreas targets only cells that play a role in building muscle protein and decreasing blood sugar, but Insulin has no effect on, say, the functions of the Ovary, whose cells have no Insulin receptors.

The Endocrine system is closely integrated with the central nervous system (CNS), and many researchers use the term Neuroendocrine System to reflect this relationship. Indeed, two of the Endocrine glands are discrete regions of the brain—the Pineal Gland and the Hypothalamus—and the Adrenal Medulla is made up entirely of neural cells (see below).

    • Most hormones are secreted into the blood stream and circulate through the body until they are recognized by receptor cells and carry out their function. This is a relatively slow process, but the hormone can travel from the secreting organ to distant parts of the body, it can affect different parts of the body simultaneously, and it can be long-lasting.
    • Some hormones are secreted directly from one gland to another through a tiny local blood capillary system, a quicker process than through general blood circulation.
    • Some glands have neurosecretory cells that produce neurohormones in response to neural stimuli from the CNS. These molecules have axons that communicate across synapses to neuroreceptors in the target tissues. This mode of communication is more immediate than that of hormones that travel through the blood but its effect is of shorter duration. The Pineal Gland, Hypothalamus, and Adrenal Medulla secrete neurohormones.

Most avian hormones work in much the same way that human hormones do.

    • They can have a positive or stimulatory effect, or a negative or inhibitory effect. They can stimulate or inhibit the synthesis and/or secretion of another hormone, or a process such as gonadal recrudescence or regression, the production of ova and sperm, or feather growth. Some induce or inhibit a particular behavior, such as nest-building, bonding behaviors between male and female, incubation or brooding, or reactions to stressful situations.
    • Some hormones are produced and secreted only at certain times during the Life History of a bird, especially during the breeding season. Others work constantly to maintain balance in essential systems like blood pressure and salt/water ratios.
    • Most hormones are regulated by a negative feedback loop, where the secreting organ receives information from other glands, organs, or the bloodstream about hormone levels and responds by decreasing or ending its secretions. For example, after eggs are laid the gonadal hormones send negative feedback to the Hypothalamus and the Pituitary Gland to decrease the hormones that trigger egg production. Some hormones produce a positive feedback loop, where an increasing blood level can lead an organ to increase its secretions. For example, a high concentration of Mesotocin, which helps induce uterine contractions, induces the Hypothalamus to continue its secretion to sustain contractions through the egg-laying.
    • Hormones can be triggered by endogenous factors (internal), such as other hormones or internal bodily processes, or by exogenous factors (external), predictable circumstances or events like the changing photoperiod (amount of daylight), or unpredictable events like territorial disputes or extreme weather.

Different species of birds may have differences in how their hormones work. The distinctive developmental processes of altricial birds like songbirds and raptors vs. precocial birds like waterfowl, for example, are marked by significant differences in the timing and functioning of hormones in both growing and mature birds. Likewise, the hormonal patterns of large birds like Bald Eagles who usually produce only one clutch of eggs each year, and of songbirds who can produce two or more egg clutches each year, vary in number, order, and duration.

The amount of research on the Endocrine system in birds is enormous, but there is still much that is not known. Until recently research has focused on domestic poultry, but an increasing body of research is being conducted on other species, especially songbirds and waterfowl. Relatively little experimental work has been done with raptors. What follows is largely a description of the system as it is known for birds in general, with occasional comments pertaining to raptors or specifically Bald Eagles if known.

THE GLANDS AND THEIR HORMONES

This schematic diagram shows the location of the Endocrine glands and other organs that secrete hormones (adapted from N.S. Proctor & P.J. Lynch, Manual of Ornithology: Avian Structure & Function, Yale University Press, 1993 and used by permission of the publisher). Click on the image for an enlarged view.

The tiny Pineal Gland is located at the top of the brain between the cerebral hemispheres and the cerebellum.
The Hypothalamus is a small region of the forebrain in front of the optic lobe and is connected to the Pineal by a stalk-like structure.
The Pituitary Gland is at the base of the brain immediately below the Hypothalamus. It has two discrete regions, the Anterior lobe and the Posterior lobe.
The Thymus is below the sternum at the front of the neck.
The Thyroid is located at the base of the neck.
The multi-lobed Parathyroid is at the base of the neck adjacent to the Thyroid.
The paired Ultimobranchial Glands are below the Parathyroid.
The Gonads (Ovary and Testes) are located in the abdominal cavity next to the top lobe of the Kidneys. (Bald Eagle females, like most female birds, have only the left Ovary; the right Ovary is vestigial or absent.)
The paired Adrenal Glands are in the abdominal cavity above the top lobe of the Kidneys. As in mammals the avian Adrenals have two types of tissues that produce different hormones, the Cortex and the Medulla, but in birds these tissues are not segregated from each other but are intermingled.
The Pancreas is located in the duodenal loop of the intestines.

The text that follows goes into detail about each of the glands, organs, and hormones that make up the avian Neuroendocrine system. Some readers may want to dig through the weeds, while others may think it’s more than they want to know, at least in one reading. But there are pictures! Mini-flowcharts show the paths of hormones and their effects on other hormones and glands. This stuff will always be here for anyone who wishes to use it as a reference. It can be searched for information about specific organs, hormones, and functions to suit the reader.

PINEAL GLAND

The Pineal Gland (the name comes from its pinecone-like shape in humans; it has a variety of shapes among birds) is made up primarily of specialized neural cells, or neurons, which keep all parts of the body in internal sync and the body itself in sync with the external environment. The Pineal, the Hypothalamus (see below), and in some species the Retinas of the eyes (it is uncertain whether this includes Bald Eagles), are “oscillators,” in that their activity follows a daily cycle of ebb and flow from day to night. Working together via a network of neurohormones (although the exact mechanism of their coordination is unclear), they operate as “pacemakers” for the entire body, transmitting their daily oscillation throughout the cells to synchronize all the endogenous systems to a single daily “biological clock,” the body’s “circadian rhythm” (Latin circa = about + dies = daily). (Detailed discussion of circadian rhythms and photoperiodism is here.)

The Pineal’s location at the top of the skull enables it to detect the daily oscillation between sunlight and darkness caused by the Earth’s revolution around the sun and rotation on its axis. It does this by means of photosensitive cells or photoreceptors (called pinealocytes) that perceive sunlight and measure its daily length (the photoperiod). The photoreceptors also store changes in the duration of light and dark over the course of the day and from one day to the next as the year progresses, thus deducing the approximate time of day and the season of the year. (In humans the Pineal is buried in the middle of the brain and does not detect light directly but through signals from the retinas.)

Melatonin (MEL) is the product of the Pineal’s photoreceptors. It is a neurohormone that regulates the body’s endogenous rhythms and coordinates them with the exogenous environment. MEL is inhibited by light, so it is secreted only during nighttime hours. The daily oscillation of MEL regulates the body’s circadian rhythms, and it also decreases body temperature and metabolism to decrease energy use during the dark hours. As the photoperiod changes from one day to the next, the cyclic duration and amplitude (amount) of MEL’s secretion changes. MEL communicates this daily change via axons across synapses to the Hypothalamus, which responds by sending neural signals further downstream to initiate hormonal changes appropriate to the season in the annual cycle of birds (see discussion of the annual cycle, or Life History, in this page).

In the mini-flowcharts below, orange boxes represent glands and organs, blue boxes represent hormones, green arrows represent positive effects, and red arrows represent negative effects. Click on an image for an enlarged view.

HYPOTHALAMUS

The Hypothalamus exchanges lightning-fast neural signals with the Pineal Gland, enabling them to work in concert as pacemakers to regulate circadian rhythms. Like the Pineal, the Hypothalamus has photoreceptors that respond to sunlight. As a region of the brain, it also receives information from other environmental stimuli, and thus it is a bridge between the endogenous and the exogenous. The Hypothalamus has a much broader and more diverse role than the Pineal. It monitors all the internal processes throughout the body. It also is the link between the Central Nervous System and the Endocrine System. Its neurosecretory cells produce a variety of neurohormones and secrete most of them directly via a special system of capillaries into the Anterior lobe of the Pituitary Gland (see below). It also releases two neurohormones via axons into the Posterior lobe of the Pituitary Gland to be released when needed.

The neurohormones produced and secreted by the Hypothalamus begin sequences of hormones from other organs that affect breeding behaviors, growth and metabolism, salt/water balance, responses to stress, and other essential functions. Several hypothalamic hormones are known as “releasing” hormones, in that they direct the Pituitary Gland to release hormones that trigger the production and release of other hormones.

Gonadotropin-Releasing Hormone (GnRH) is a releasing hormone that initiates a cascade of reproductive hormones at the start of the breeding season, in response to neural signals about the changing photoperiod. (See more detail here on Reproduction and Hormones, and here on Photoperiodism.) GnRH stimulates hormones (gonadotropins LH and FSH, see below) in the Anterior Pituitary to induce regrowth of the Ovary and Testes (gonadal recrudescence) and the synthesis and release of ESTR, TEST, and PROG. These gonadal hormones send positive feedback in the female that stimulates GnRH to a sudden surge just before ovulation. After a clutch of eggs is complete, secretion of GnRH decreases, causing ovarian follicles to shrink and be reabsorbed.

Gonadotropin-Inhibiting Hormone (GnIH) is an inhibiting hormone that has the opposite effect of GnRH, coming into play as the breeding season comes to a close. From ovulation to oviposition, gonadal hormones prevent the secretion of GnIH with its inhibitory effect. But after eggs are laid, GnIH suppresses the release of reproductive hormones from the Anterior Pituitary and the Gonads.

Corticotropin-Releasing Hormone (CRH) is a releasing hormone that initiates hormonal and metabolic responses to stressful events or circumstances, such as injury, territorial challenge, bad weather, scarce food, human disturbance, toxins, or disease. It initiates a series of hormones from the Anterior Pituitary to the Adrenal Cortex and the Thyroid that redirect the body’s energies to effective responses, including increased levels of blood glucose, fatty acids, and muscle proteins, and enhanced metabolism. It also decreases gonadal hormones and digestive processes.

Growth Hormone-Releasing Hormone (GHRH) is a releasing hormone that starts a cascade of hormones for growth, metabolism, and development in young birds. Its concentration is higher in embryos and young birds than in adults, and it declines gradually as birds grow. The low plasma amount in adults can aid in the breakdown of lipids and increase blood glucose in response to stress events.

Thyrotropin-Releasing Hormone (TRH) is a releasing hormone with a role in promoting growth and development, and metabolism in young birds by helping increase metabolism and heat production. It can have that same effect in adults, especially in response to cold weather, and it plays a role in initiating growth and functions of the gonads at the start of the breeding season.

Somatostatin (SS) is an inhibiting hormone that responds to stress by suppressing hormones that promote metabolism and growth. It diverts energies from breeding behaviors and feather growth. SS is also secreted by the Pancreas (see below), which helps regulate blood sugar levels.

Vasoactive Intestinal Peptide (VIP) has the principal role of regulating the body’s salt/water balance (see below under Gastrointestinal Tract hormones). VIP produced in the Hypothalamus is a releasing hormone (like GnRH), triggered at the beginning of the breeding cycle as the ovarian follicles start to grow. It is the main factor in stimulating the secretion of PRL, which leads to growth of the most mature ovarian follicles (preovulatory or hierarchical follicles) and suppresses growth of the less mature ones (prehierarchical). In young birds it helps stimulate metabolism and growth.

Arginine Vasotocin (AVT) is the main antidiuretic in birds (analogous to mammalian Antidiuretic Hormone), responding to excessive heat and other stressors, and to low water levels in the blood. AVT is secreted via axons into the bloodstream to the Posterior lobe of the Pituitary Gland to be stored and released into the bloodstream when needed. When the Hypothalamus detects low water concentration in the blood, it sends AVT to the Kidneys to instruct them to absorb and conserve water. AVT also has a crucial role in the egg-laying process. It is stimulated by ESTR, PROG, and MT to surge to a peak just before oviposition, when it relaxes the vagina and sphincter and induces uterine contractions. As ovulation and oviposition come to an end, AVT helps inhibit the secretion of TEST to decrease courtship behavior and stimulate the secretion of PRL to induce parental behavior.

Mesotocin (MT) (roughly analogous to mammalian Oxytocin) is secreted via axons to the Posterior lobe of the Pituitary Gland for storage and release into the bloodstream when needed. It activates the release of AVT to induce uterine contractions, and as the blood level of MT remains high it exerts positive feedback to intensify the contractions until the egg is laid. MT helps inhibit the gonadal hormones as each egg is laid. MT also helps lower blood pressure by its effect on Adrenal hormones (see below). MT can be inhibited by excessive environmental heat, thus possibly delaying oviposition.

PITUITARY GLAND

The Pituitary gland sometimes is called the “master gland” because its hormones regulate the production and release of downstream hormones that are involved in myriad bodily functions, including reproduction, growth and metabolism, salt/water balance and blood sugar regulation, stress responses, feather growth, molt, and migration. It serves as a gatekeeper, sorting out the multifarious signals it receives from throughout the body and from some exogenous cues, making sure that all the hormonal secretions and endogenous processes occur at the right time.

ANTERIOR LOBE of the PITUITARY GLAND

The Anterior lobe receives neural signals from the Hypothalamus which trigger the production and release of hormones that regulate processes of the Gonads, Adrenal Glands, Thyroid, Pancreas, and Stomach.

Luteinizing Hormone (LH) is stimulated by GnRH from the Hypothalamus as the breeding cycle begins. (See more detail here on Reproduction and Hormones.) It spurs the Gonads to grow and to release their hormones, stimulates the production of sperm and the rapid maturation of preovulatory follicles, and increases lipid metabolism in the liver and induces the deposition of yolk from the liver into the ovarian follicles. LH responds to positive feedback from PROG and ESTR, and it surges just before ovulation of the most mature ovarian follicle (F1). After all eggs are laid, INH inhibits the secretion of LH. Loss of a brood of chicks can result in increased secretion of LH to induce a new breeding attempt.

Follicle-Stimulating Hormone (FSH) is triggered by GnRH to stimulate the growth of the Gonads and release of gonadal hormones. It stimulates nest-building, especially in the female, and helps induce the production of sperm in the male. Like LH, it induces yolk deposition in the first ovarian follicle in line (F1) and stimulates its maturation. FSH also stimulates the release of INH (see below), which suppresses development of the less mature ovarian follicles (prehierarchical). In the female FSH rises a few hours before ovulation and increases after ovulation, promoting cell growth in the embryo, then decreases as incubation begins. FSH receives positive feedback from the gonadal hormones, and after incubation begins it sends negative feedback to the Hypothalamus, which thus inhibits secretion of ESTR and PROG, causing the ovarian follicles to shrink and be reabsorbed into the body.

Prolactin (PRL) regulates parental behavior from ovulation through chick-rearing—in many altricial birds including Bald Eagles its levels remain high through chick rearing up to fledges. It is stimulated by VIP from the Hypothalamus, increasing just before ovulation. With ESTR (see below), PRL helps induce formation of the brood patch. It is stimulated by the presence of egg(s) in the nest. After eggs are laid it sends negative feedback to the Hypothalamus to inhibit secretion of GnRH, thus inhibiting the secretion of LH, FSH, ESTR, and PROG and shutting down ovulation as incubation begins. It remains high during hatching, and it peaks as it induces the beginning of gonadal regression. It then gradually declines through the chick-rearing period and facilitates the start of molt. If incubation or brooding is interrupted by stressful events, such as territorial challenges or loss of a nest or eggs, PRL decreases, making possible a new breeding attempt. Stress events depress PRL levels while increasing CORT from the Adrenal Cortex  in response.

Adrenocorticotropic Hormone (ACTH) has the role of managing the body’s responses to stress. When the Hypothalamus receives and processes information from disruptive factors, it sends CRH to the Anterior Pituitary to trigger secretion of ACTH. This hormone induces the secretion of Adrenal hormones, which in turn either stimulate (e.g. CORT, EPI, and NE) or inhibit (e.g. gonadal hormones) other hormones to marshal the body’s resources and processes as needed to meet the challenges. It also can help induce incubation behavior and migration restlessness. ACTH can reduce resistance to disease.

Growth Hormone (GH) is essential to growth and development in embryos and young birds, where it stimulates cell division, growth, fat synthesis and metabolism, glucose levels, development of the immune system, protein synthesis, and bone growth. GH peaks in the late embryo, increases gradually in growing chicks, then declines after juvenal feathers emerge. Adults have low levels of GH, but it can stimulate hormones that break down lipids and increase blood glucose in response to stressors.

Thyroid-Stimulating Hormone (TSH) manages hormone secretions for metabolism and growth, especially in young birds. TSH is triggered during excessive cold and inhibited by excessive heat and food deprivation. It stimulates release of PRL and of T4 and increases blood plasma levels of T3.

POSTERIOR LOBE of the PITUITARY GLAND

As explained earlier, the Posterior lobe does not produce hormones. It receives and stores two hormones produced in the Hypothalamus (see above), the antidiuretic AVT which induces uterine contractions, and MT which activates the release of AVT. When these are needed, the Hypothalamus sends neural signals to the Posterior lobe to release them.

OVARY AND TESTES (THE GONADS)

The primary function of the Gonads is to produce ova and sperm, and their hormonal secretions play a central role in those processes, as well as in controlling reproductive behaviors. The Gonads produce steroid hormones, which act slightly more slowly than the peptide hormones of other glands, but their effect is more prolonged. The gonadal hormones are released slowly at first but accelerate and then peak near the apex of the reproductive process as eggs are laid, then decline rapidly. The timing for their release in male and female birds can differ. When blood plasma levels of these hormones reach a certain level, they send negative feedback to the Hypothalamus and the Anterior Pituitary to suppress their stimulatory hormones. (See more detail here on Reproduction and Hormones.)

Testosterone (TEST) is the chief Androgen, produced by the Testes and in smaller amounts in the Ovary. It is stimulated by LH and FSH. It stimulates the recrudescence of the Testes and the production of sperm, and contributes to regrowth of the oviduct, development of ovarian follicles, and development of eggs. The peak synthesis of sperm is timed to coincide with ovulation and fertilization in the female. The small amount of TEST secreted in the Ovary stimulates the growth of the most mature preovulatory follicle (F1), and it peaks a few hours before ovulation. Ovarian TEST also may help induce the liver to synthesize yolk components and of albumen in the egg. TEST inhibits molt by suppressing the secretion of T3 and T4. In Bald Eagles molt begins about halfway through the incubation period, suggesting a steep decline in secretion of TEST after oviposition.

Estradiol (ESTR) is the main Estrogen in birds, produced by the Ovary and in smaller amounts in the Testes. In females, it induces nest-building, stimulates recrudescence of the Ovary and growth of the oviduct, and contributes to development of her brood patch by stimulating the Ovary’s secretion of PROG (see below). It induces synthesis of yolk proteins in the liver for deposition in the ovarian follicle and the synthesis of albumen. ESTR stimulates maturation of preovulatory follicles, peaking a few hours before ovulation, when it sends positive feedback to the Hypothalamus, resulting in a sudden surge of GnRH, LH, and FSH to induce ovulation. A few days before ovulation ESTR stimulates calcium metabolism necessary for shell production in the oviduct. It helps stimulate release of AVT from the Posterior Pituitary to induce uterine contractions. ESTR suppresses secretion of the inhibiting hormone GnIH from the Hypothalamus in the period between ovulation and oviposition. In males, ESTR helps increase the maturation of the Testes, leading to sperm production. Once a clutch of eggs is complete ESTR inhibits GnRH and then decreases significantly and can thus help initiate molt.

Progesterone (PROG) is secreted from the preovulatory ovarian follicle (F1) and, in smaller amounts, from the Testes. In males it may help stimulate nest-building. In females it aids defeathering and vascularization of the brood patch. PROG may trigger release of sperm stored in the oviduct at the uterovaginal junction, enabling them to travel upward to the infundibulum of the oviduct where one can fertilize the ovum. PROG peaks a few hours before ovulation. It can stimulate CORT, which can help induce the sudden preovulatory surge of LH that helps induce ovulation. PROG stimulates the release of AVT from the Posterior Pituitary to induce uterine contractions. It begins to decline a few hours after ovulation. After a clutch of eggs is complete, PROG inhibits secretion of GnRH, LH, and FSH. Along with ESTR, it may play a role in gender determination of the F1 ovum.

Inhibin (INH) is secreted by the Testes and the Ovarian follicles, especially F1. Its secretion ebbs and flows through the egg-laying period. It plays an essential role in ovulation, complementing the action of FSH so that only one egg ovulates at a time. As breeding begins, LH suppresses INH to prevent it from inhibiting the growth of mature (hierarchical) follicles. As the first ovulation of a clutch approaches, FSH induces maturation of the most mature ovarian follicle (F1) while it stimulates secretion of INH, which suppresses maturation of less mature follicles. After one ovulation and fertilization, INH secretion decreases significantly, allowing FSH to stimulate maturation of the next follicle in line, then it begins to rise again to target the less mature follicles. After a full clutch of eggs is laid INH peaks and inhibits synthesis of GnRH, LH, and FSH, which also results in suppression of gonadal hormones, then it declines. After a clutch is complete INH may have a role in stimulating secretion of PRL by the Anterior Pituitary, inducing parental behavior.

Vasoactive Intestinal Peptide (VIP), mainly a hypothalamic hormone involved with helping maintain the body’s salt/water balance, is secreted in small amounts by the Ovary at the beginning of the breeding cycle, stimulating nest-building behavior in the female and helping stimulate growth and differentiation of the preovulatory follicle (F1).

ADRENAL GLANDS

When the Hypothalamus perceives unpredictable and stressful exogenous events such as bad weather, injury, disease, poisoning, food deprivation, territorial disputes, and human disturbance, it rapidly signals the Adrenal Glands to effect the body’s response. Adrenal hormones regulate multiple processes throughout the body and influence a wide variety of behaviors. The Cortex tissues secrete steroid hormones into the bloodstream. The Medulla tissues are neurosecretory cells connected directly to the central nervous system (CNS) through neurotransmitters from the spinal cord. The Medulla and the Cortex respond to the same triggers, but the neurohormones of the Medulla act faster than the steroid hormones of the Cortex (within seconds as opposed to minutes). The Medulla hormones are inactivated sooner after a crisis dissipates than those of the Cortex.

ADRENAL CORTEX

The Cortex secretes glucocorticoids, whose function is to regulate glucose levels for adequate energy flow, and mineralocorticoids, whose role is to maintain an optimal salt/water balance.

Corticosterone (CORT) is the avian body’s chief glucocorticoid, which helps increase and optimize the body’s use of energy in response especially to stressful events or circumstances. It affects nearly every part of the body at one time or another and interacts with numerous other organs and hormones. CORT increases blood flow to muscles and breaks down their protein to amino acids for conversion to glucose. It increases production of lipids and their deposition in the liver. It stimulates food intake for immediate energy and suspends digestion and growth factors that could interfere with immediate metabolic needs. CORT suppresses gonadal hormones, resulting in interruption of breeding activities, which could include delay of oviposition. CORT enhances memory of stressors so they can be anticipated and avoided, and even can increase sensitivity to possible impending stressors. A prolonged stressful situation and the resulting extended secretion of CORT can result in decrease in body weight, resistance to disease, or abandonment of nest, eggs, or chicks.

Aldosterone (ALDO) is a mineralocorticoid that helps raise blood pressure and blood salt level. It is stimulated by ACTH and inhibited by MT. ALDO stimulates the Kidneys to release REN to enhance reabsorption of sodium. It declines after hatch into adulthood.

Cortisol is a glucocorticoid that is abundant in embryos and early hatchlings, where it is essential to development of organ systems. It declines in adults, although it may have some role in the functioning of the immune system.

The Adrenal Cortex also secretes small amounts of ESTR, which in the female helps regulate blood calcium, and PROG, which may stimulate the preovulatory surge of LH leading to ovulation and may play a role in shell formation. The Cortex also secretes a small amount of INH after ovulation to stimulate development of the embryo. The low amount of Adrenal TEST secreted in the male supplements gonadal TEST.

 ADRENAL MEDULLA

The central nervous system responds immediately to emergency situations by accelerating the heart, dilating the bronchi, and suppressing digestion, among other effects. Its neural signals to the Medulla trigger secretion of neurohormones that signal other organs via the blood stream to act. These responses are quick, producing the so-called “fight or flight” response, but they are short-lived.

Epinephrine (EPI, also referred to as Adrenaline) and Norepinephrine (NE, also referred to as Noradrenaline) have many of the same effects as CORT in response to stress, including increasing and helping maintain the metabolic rate, blood sugar, blood fat, and blood pressure. But as neurohormones rather than steroids, EPI and NE work faster than hormones from the Adrenal Cortex. They increase blood to the muscles, increasing their strength and enhancing their response, and to the brain, increasing neural activity. They suppress digestion and help regulate thermogenesis in cold weather. They also lower resistance to disease. EPI and NE inhibit secretion of INS from the Pancreas to help maintain blood pressure. EPI reduces breeding activities and can delay oviposition. NE is produced also in the brain, where it plays a role in regulating circadian rhythms by inhibiting the synthesis of MEL. (See more in Photoperiodism.) Birds may have a greater amount of circulating NE than of EPI.

Vasoactive Intestinal Peptide (VIP) produced in the Medulla helps induce responses to stress by stimulating the release of other hormones, like EPI, NE, TEST, and CORT. VIP also is secreted by the Hypothalamus and Ovary during the breeding cycle (see above) and the Intestines to help regulate salt/water balance (see below).

Atrial Natriuretic Peptide (ANP) from the Medulla has an inhibitory effect on the secretion of other Adrenal hormones. ANP also is a secreted by the Heart, acting as a diuretic to lower blood pressure (see below).

THYROID GLAND

The two hormones that the Thyroid Gland secretes play essential roles throughout the body in maintaining optimal metabolism and regulating body temperature, oxygen consumption, heart rate, blood sugar, gonadal recrudescence, production of ova and sperm, egg-laying, and feather growth, development, and pigmentation. The timing of their actions is complementary: T3 surges in daylight and recedes at night, while T4 recedes at night and surges in daylight. An adult female must have ample levels of Thyroid hormones of her own so that she can deposit enough in the yolk to supply the those in the Thyroid of the embryo to optimize cell differentiation and maturation. Thyroid hormones surge in late embryos, stimulating their development. In young altricial birds like Bald Eagles the hormones increase through the early hatchling period and induce growth and development, especially of feathers and muscles, and they are essential for development of thermoregulation, imprinting, and learning skills. In adults, both hormones exert negative feedback on reproductive hormones to redirect energies toward metabolism when needed. The two hormones serve different functions during the breeding season.

Triiodothyronine (T3) is critical to the growth and development of embryos and young birds, after which it declines and remains at a low level through adulthood. T3 is stimulated by TSH, cold weather, and environmental contaminants. Some T4 is converted to T3, increasing its blood plasma level and leading to increased secretions of reproductive hormones in adults, which helps spur gonadal growth and production of ova and sperm during the breeding season. Hot weather and food deprivation can inhibit the secretion of T3.

Thyroxine (T4) is present in greater quantity than T3 in birds. It is stimulated by GH and TSH, cold weather (but less so than T3), and food deprivation. GH increases the conversion of T4 to T3, which is critical to maintaining T3‘s concentration. T4 controls bodily growth and development. T4 increases as the breeding season starts and stimulates secretion of gonadal hormones, peaking during the copulation period. Later it induces the onset of molt and the growth of new feathers. Gonadal hormones and hot weather inhibit T4.

ULTIMOBRANCHIAL GLANDS

Calcitonin (CT) (which in humans is produced and secreted by the Thyroid) responds to excessively high blood calcium levels, having the opposite effect of PTH from the Parathyroid Gland. It causes calcium to be taken out of circulation and deposited in the bones. In breeding females CT peaks just after ovulation, then falls significantly to allow blood calcium to go to the Uterus during shell deposition rather than to the bones, and it rises after the shell is calcified.

PARATHYROID GLAND

Parathyroid Hormone (PTH) is triggered by low blood calcium levels, having the opposite effect of CT from the Ultimobranchial Glands. It stimulates the Kidneys to reabsorb calcium from the small intestine so it is not excreted from the body, and it activates vitamin D which causes bones to release calcium into the blood. In breeding females PTH secretion increases after ovulation and is at high levels during shell production in the Uterus.

PANCREAS

The Pancreas secretes digestive enzymes to the small intestine and hormones to regulate glucose, metabolism, and protein and lipid metabolism. Its hormones interact with each other, maintaining a balance in the various processes.

Glucagon (GLUC) is the principal pancreatic hormones in birds, found in higher amounts than in mammals. It is stimulated by other pancreatic hormones, GH, and food deprivation. GLUC causes the liver to break down lipids and release fatty acids to the blood, thus counteracting the effect of INS. It breaks down muscle protein to release energy, increases blood sugar, and regulates heat production in cold weather.

Insulin (INS) is essential in building muscle protein and counteracts the effects of GLUC in the liver. Birds have lower levels of INS than of GLUC. INS decreases blood sugar, increases blood fat, stimulates protein synthesis and the production of fatty acids in the liver, and helps regulate heat production in cold weather. It inhibits hypothalamic secretions in order to decrease Thyroid hormones. In embryos, INS stimulates metabolism and growth, especially of skeletal muscle. INS is inhibited by pancreatic SS and EPI.

Somatostatin (SS) responds to stress and is found in higher amounts in birds than in mammals. It increases blood sugar, corticosteroids, and fat stores. It inhibits the secretion of VIP from the intestines, suppressing the release of stomach acids and digestive enzymes and decreasing intestinal absorption of glucose and lipids. SS stimulates GLUC and inhibits INS to keep them in an optimal ratio. In young birds SS diverts energies from growth and metabolism, while in adults it suspends breeding behavior and feather growth. A small amount of SS is secreted also by the Hypothalamus (see above).

THYMUS

Thymosin hormones aid in the development of the immune system in the young and the synthesis of T-lymphocytes and infection antibodies.

HEART

Atrial Natriuretic Peptide (ANP) is a diuretic that lowers elevated blood salt and blood pressure, thus counteracting the effect of REN from the Kidneys. It signals the Kidneys to increase excretion of sodium and the Parathyroids to decrease absorption of calcium. It also has an inhibitory effect on some pituitary and adrenal secretions (see above).

KIDNEYS

Renin (REN) helps raise low blood salt and blood pressure, thus counteracting the effect of ANP from the Heart. It induces synthesis of ANG and ALDO.

Erythropoietin (EPO) is secreted by the Kidneys in response to T4, stimulating production of red blood cells.

LIVER

Angiotensin (ANG) is signaled by the Kidneys to raise blood pressure by inducing secretion of ALDO from the Adrenal Medulla.

GASTROINTESTINAL TRACT

Ghrelin (GHRL) is secreted by the Stomach and helps lower the body’s fat levels. It stimulates secretion of GH to activate the breakdown of lipids. GRHL may suppress hunger and reduce fat stores in the body.

Vasoactive Intestinal Peptide (VIP) synthesized in the Intestines plays the principal role in regulating the body’s salt/water balance. It also induces relaxation of digestive muscles and aids in the absorption of glucose and lipids. (See above for discussion of VIP’s secretion from the Hypothalamus, Ovary, and Adrenal Medulla.).

References: Endocrine System – Hormones