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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 3^rd 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 2^nd, 3^rd, and 4^th 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