Tag Archives: egg failure

WHEN BALD EAGLE EGGS DON’T HATCH

© elfruler 2013, 2024
Click here for full citations of References cited on this page.

Data collected from 2006-2020 from Bald Eagle video cameras yield a sizable body of statistics about eggs, hatches, and fledges. Discussion of these data and several Tables that summarize them can be accessed here. Over the 15-year period, 20.8% of the eggs laid at these nests were lost or never hatched. This falls within the range of 10%-25% of unhatched eggs that is suggested in published research. This Table summarizes the numbers of failures and what is known about their causes.

External events like intruders, predation, weather, abandonment, fallen nests, and accidents might lead to the loss of eggs. As Table 3 shows, such circumstances account for about 28% of the lost eggs. Events like this are often observable on a nest cam and are not addressed here. Other causes listed in the Table – unhatched eggs, broken eggs, eggs that disappeared, hatching failure, and reasons that are simply unknown – incorporate about 67% of the losses. In such cases, the cause usually is impossible to determine.

If an egg remains unhatched, it is either unfertilized (sometimes referred to as infertile) or nonviable (or inviable). Infertility is an issue concerning the reproductive processes of one or both parents. Nonviability (not able to live or survive) is an issue with the development of the embryo. In only about 5% of the losses in the Table were eggs determined with certainty to be infertile or nonviable. This page explores what might cause infertility and nonviability.

Several pre-disposing factors can lead directly to egg failure, or they can bring about other circumstances that themselves are the direct cause of loss. “Ultimate causes,” as some scientists have called them (see Newton 1979; Newton 1993), are not always easy to observe from a video camera, but they can include:

    • Inadequate food supply, which is unlikely at the start of a breeding season because Bald Eagles choose their nesting territories with care, but food can become scarce or harder to find as a result of bad weather, human activity, or other overwhelming events.
    • Weather, including extreme temperatures, storms, and persistently excessive high or low humidity.
    • Territorial intrusions and predation by other Bald Eagles (“intraspecific intrusions”), Great Horned Owls, Barred Owls, Common Ravens, American Crows, Black-Billed Magpies, foxes, raccoons, bobcats, and bears.
    • Human activity, including habitat destruction, introduced contaminants (pesticides, herbicides, rodenticides, industrial chemicals, etc.), disruptive proximity to nests, and poaching. (Newton 1979)
    • Bacteria, fungi, and other micro-organisms, which can cause disease or infections. (Houston et al. 1993; Cooper et al. 1993; Cook et al. 2003)
    • Age of one or both adults, either youth or senescence (a decline in reproductive success as a result of aging).

Ultimate factors often lead to secondary circumstances, or “proximate causes,” that result in loss of an egg. For example,

    • A territorial challenge or inadequate food supply might lead parents to abandon a clutch of eggs.
    • Bad weather might cause a nest tree to fall, destroying the eggs.
    • High humidity can create a greater risk of bacterial infection.
    • Catastrophic events like bad weather and intrusions can make foraging more challenging for the adults, who may be forced to spend more time seeking food, or even abandon the eggs altogether in order to survive. Such events also can result in loss of one of the adults, greatly increasing the cost of incubation to the remaining mate. Inconsistent incubation can expose eggs to predation or to extreme weather that can lead to impaired embryo development, hatch failure, or death. An increase in the incubation period can diminish the condition of the chick at hatch. (Reid et al. 2002)
    • Human activity can disrupt the fitness and breeding activities of the adults, interfering with egg fertilization or embryo development.

Infertile eggs

An egg is considered infertile if the ovum in the female’s oviduct is not fertilized by the male’s sperm. (Note that the female herself is fertile, by virtue of her laying an egg.) Among the lost eggs at nests with cams, only 2 (1.1%) were collected and verified by laboratory examination to have been infertile. There are several possible reasons for infertility:

    • External circumstances. Bad weather, the presence of an intruder, human activity, or an inadequate food supply can disrupt the reproductive hormonal cycle.
      • Extreme cold can reduce the number of sperm and ova available (Evans & Heiser 2004).
      • Stressful events like intrusions or human disturbance cause the release of adrenal hormones (Corticosterone, Epinephrine, Norepinephrine), which induce the eagle to devote its energies to responding. These hormones suppress the reproductive hormones, which can decrease the production of gametes or throw the hormonal cycles of the mated pair out of sync. (See more on hormones here.) Bald Eagles are learning to adapt to urban habitats. They are more sensitive to human disturbance early in the breeding season than later. (Newton 1979)
      • Pesticides, herbicides, rodenticides, industrial and agricultural chemicals can disrupt hormonal and reproductive processes, affect the viability of eggs (more than their fertility, see Newton 1979), or damage essential organs or metabolic systems (Newton 1979; Newton 1993; Weidensaul 1979; Weidensaul 1996; Ottinger 2015).
    • Poorly timed copulation. If insemination occurs more than about a week before an ovum has been released from the ovary, the viability of the sperm decreases even though it can still be stored in the oviduct. (Heidenreich 1997) On the other hand, if insemination occurs more than about 4 hours after an ovum is released into the oviduct, the yolk and embryo have moved into the magnum region of the oviduct, where albumen is added, then the isthmus region where shell membranes are added, which the sperm cannot penetrate. Also, sperm are at their highest concentration early in the breeding season (Blanco et al. 2007). Poor timing can occur because of:
      • A newly formed pair. Even among experienced adult Bald Eagles, a new bond usually takes several weeks to develop, and the hormonal secretions of the two might not be timed properly to bring gamete production into sync. (See more discussion here.)
      • An external disturbance, such as human activity or intruders. If extensive, such events can disrupt the eagles’ hormonal cycles or copulation activity.

Frequent copulation increases the odds for successful timing (Fox 1995), but this does not always compensate for other factors that can prevent successful fertilization.

    • Young age. There have been recorded instances of a 4-year-old Bald Eagle successfully breeding, but there are many examples of unsuccessful breeding by pairs in which, for instance, the young male’s sperm are not produced in sufficient numbers to achieve fertilization, or either the male or the female is a new breeder whose gamete production is not in sync with its partner’s. Young eagles also might be clumsy in copulation, failing to attain the so-called “cloacal kiss” or direct contact between the cloaca of male and female to successfully release sperm into the oviduct. (Fox 1995)
    • Old age. Senescence can decrease gamete production in both males and females. It can also result in soft eggshells. (Cooper 2002)
Nonviable eggs

An egg is considered nonviable, sometimes called addled or rotten, if the embryo of a fertilized egg fails to develop properly during incubation and dies. Only 7 (3.7%) of eggs lost at nests with cam were known to have been nonviable. Nonviability can have many causes, among them:

    • Insufficient egg turning. Turning the eggs is essential during incubation, for different reasons through the incubation period. During roughly the first half of the incubation period, the adults generally turn the eggs every 20 minutes to an hour. The most crucial time is the first third of the period, days 1-12 for Bald Eagles. (Deeming 1989a, 1989b, and 1989c; Fox 1995; Carey 2002; Deeming 2002c; Ar & Sidis 2002; Deeming 2009) Raptors in general are highly attentive and consistent incubators (see Deeming 2002b), but various circumstances can interrupt their faithful egg turning.
      • While it is commonly asserted that egg turning keeps the embryo from adhering to the extra-embryonic membranes, studies have shown that this has little effect on the hatchability of eggs. (Deeming 1989a; Deeming 1989b; Deeming 2009) Likewise, even distribution of heat throughout the egg is not a principal reason for turning, as demonstrated in artificial incubation operations where the equipment provides heat on all sides of the egg, yet it still requires turning to develop properly.
      • Instead, the reasons for egg turning have to do with the proper functioning of critical components in the egg that enable the embryo to develop:
        • Turning stimulates the capillaries in the yolk sac membrane to develop evenly so that sufficient nutrients can be transferred from the yolk to the embryo. Turning also assures full development of the shell membranes so that they function properly in exchanging oxygen from the outside and carbon dioxide from the inside of the egg, and in diffusing water vapor out. Studies have shown that if these capillaries line less than 90% of the shell, the embryo will have less than a 14% chance of hatching.
        • Turning assures that the yolk and embryo come into contact with fresh stores of water from the albumen necessary for the formation of the extra-embryonic fluids (amniotic and allantoic); the amniotic fluid in its turn transfers albumen proteins to the embryo that are crucial for its growth and development. Insufficient egg turning has been shown to retard embryonic growth.
        • Turning may help correct for possible twisting of the chalazae (the cords of protein that suspend the yolk and embryo in the albumen) that could interfere with keeping the embryo positioned at the top of the egg near the brood patch. (Sharpe)
      • Through roughly the second half of the incubation period, the developing embryo fills the shell and no longer floats around freely but settles into a position on its side, with its head near the air cell (Fox 1995). (Click here for discussion of hatching position.) As the parents move about in the nest, the unevenly weighted egg can shift in the nest cup, and the chick might end up lying head down, which makes hatching difficult or impossible.
        • The parents nudge the eggs periodically to reorient them so that the chick is back on its side in proper hatching position.
        • Moving the eggs also releases any friction among the eggs or with nesting material that might prevent the eggs from rolling back into the right position. (Drent 1973; Drent 1975; Fox 1995; Deeming 2002c)
        • Turning might also stimulate the pulse of the growing embryo. (Deeming 1989b)
      • Inadequate food supply to the adults before oviposition (egg-laying). Insufficient or an imbalance in nutrients in food ingested by the parents can be imparted to the embryo and arrest its normal development. The diet and health of the female in the days leading up to ovulation and during the roughly 3 days when the egg is moving through her oviduct are critical. She needs extra fat to produce a high quality yolk. Insufficient calcium and vitamin D3 in the diet can result in soft eggshells. (Cooper 2002) The female must have ample levels of Thyroid hormones of her own so that she can transfer them to the yolk to supply the embryo with enough to grow properly.
      • Hypothermia or hyperthermia. Temperature during incubation is a complex topic that involves much more than the ambient air temperature or time parents are on or off the eggs. (Drent 1973; Drent 1975; Deeming 2002a; Deeming 2015) The temperature of the embryo inside the egg is what determines whether the egg is in danger, but this is impossible to measure from a video cam. The following observations provide some general information about incubation temperature.
        • Hypothermia might seem to be the greater danger to an embryo, since so many Bald Eagles breed in temperate zones from winter to spring when ambient temperatures can be well below freezing for extended periods. The egg contents can begin to freeze if the egg’s temperature (not the ambient air temperature) descends below 0°C (32°F). Wind can make the air even colder than a thermometer records. (Huggins 1941)
        • In fact, hyperthermia is much more likely than hypothermia to be fatal to the embryo. An internal temperature above 41°C (105.8°F) will kill the embryo. (Fox 1995, 95) Prolonged exposure to excessive heat can cause eggshells to be too thin and collapse before hatching. (Cooper 2002) Direct sunlight can raise the egg temperature to a lethal degree within a few minutes. (Carey 2002)
        • The optimal internal temperature for normal embryo growth and development can range from about 32°-38°C (89.6-100.4°F). The egg’s temperature can fluctuate up and down outside of this range without adverse affects, so long as the parents are able to bring it back to an acceptable range before any damage is done. (Snelling 1972)
        • Bald Eagles generally are successful in keeping the eggs within an acceptable temperature range. They are able to sense the temperature of the egg through their brood patches, and they are aware of the ambient temperature and when the egg needs to be protected from temperature extremes. They develop an incubation “rhythm” of time on and time off the egg to control the amount of time it is exposed. (Haftorn 1988; Lea & Klandorf 2002; Hainsworth & Voss 2002) But challenges such as prolonged extreme ambient temperatures, intruders, or human disturbances can impede their ability to maintain optimal egg temperatures, especially if the food supply is disrupted or the eagles are unable to forage sufficiently to maintain their own health.
      • Humidity. There must be a proper balance of water among albumen, yolk, membranes and embryo throughout the incubation period. As the embryo develops, metabolization of the yolk and albumen produces water vapor, which along with carbon dioxide is diffused to the outside via the shell membranes. A certain amount of water loss from the egg is crucial to the environment within the egg and thus the health of the embryo. This is affected by the amount of moisture, both from rain and snow, and also from ambient humidity in the air and the humidity level in the nest. Humidity itself is affected by the ambient temperature. A higher temperature can cause more water loss, while a lower temperature can result in less water loss. Incubating adults can help keep the humidity level near the egg in balance. (Ar & Sidis 2002)
        • Low ambient humidity can result in excessive water loss from the egg, causing dehydration and drying out of the membranes or albumen, which can prevent normal embryonic development, interfere with successful hatching, or even cause death. (Cooper et al. 1993; Carey 2002; Cooper 2002)
        • High ambient humidity can lead to insufficient water loss from the egg, which can cause the embryo to suffocate or drown in excess fluids. Too much water can impinge on the space that the air cell needs to occupy during hatching, and it can interfere with the embryo’s full absorption of the yolk sac before hatching, possibly resulting in death. (Cooper 2002) Inadequate water loss could also cause the embryo to shed less weight than is necessary (10-20% of its initial mass), making it too cramped inside the shell for it to position itself for a successful hatch. (Fox 1995) Moisture also can encourage bacteria to proliferate in the nest and possibly penetrate the eggshell.
        • Bacteria, fungi, and contaminants.
          • Toxic organisms can proliferate in humid environments (see above), and they might penetrate the eggshell and damage or kill the embryo (Cooper 2007; West et al. 2015).
          • Foraging adults can pick up contaminants which as they accumulate in the adults’ bodies can affect egg production and embryo development or be fatal to the embryo (or the adults). (Blanco et al. 2007; Henny & Elliott 2007) Raptors are especially at risk because they consume prey in which a contaminant may have accumulated link by link from smaller organisms to larger ones up the food chain, until it reaches a lethal level in the immediate food source of an apex predator. (Weidensaul 1996) Bald Eagles are particularly affected because of their preferred diet of fish, which accumulate contaminants in higher concentrations than other animals (Newton 1979).
              • Chlorinated hydrocarbons used as pesticides can end up in the eagles’ food supply. DDT, which is still residual in the environment in some parts of the U.S., breaks down in the body into the metabolite DDE, which in the female prevents the metabolism of calcium that is essential to eggshell production, resulting in a thin shell that may break during the incubation period. It can also reduce the amount of calcium supplied to the embryo as its bones grow, and it can impair gas and water vapor exchange through the shell. DDE may even kill the embryo outright. PCBs also can damage the embryo.
              • Mercury can adversely affect hatching success (Newton 1979).
            • The shell is cracked or broken before the embryo has fully developed inside. The shell becomes thinner and more fragile as the incubation period proceeds because the chick absorbs some of the calcium into its developing bones. If an incubating parent moves suddenly in response to an unexpected event like an intruder or human disturbance, it might breach the shell. Parental missteps are rare, even in situations that appear on cam to be violent.
Hatching failure

Another cause of egg loss is hatching failure, when a chick begins the hatching process but dies before it is able to fully emerge. Hatching is strenuous work that requires intense effort. Several of the risks described above can make hatching difficult or impossible. These include:

    • Weakened embryo because of poor nourishment of the adults before oviposition (egg-laying), or insufficient egg-turning during incubation.
    • Bacteria or chemical contaminants that seep in through the cracked shell and deplete the chick’s strength and energy or kill it;
    • Low humidity that cause shell membranes to dry out and stop blood flow to the hatching chick or stick to or wrap around the chick so that it cannot break through;
    • Excess humidity, which can prevent necessary loss of water vapor and drown the chick;
    • Malposition of the body, which can prevent rupture of the air cell, impede the chick’s ability to peck at the shell with its egg tooth, or cause the chick to drown in fluids or suffocate in matted nesting material;
    • A false step by parent or already hatched sibling that can breach the cracked shell before the hatching process is complete.

Other circumstances that can lead to hatching failure are:

    • Rupture of the chorioallantoic membrane that lines the eggshell, which can damage the capillaries embedded in it and cause blood loss, or can lead to infection. (Snelling 1972, 1303) This may be one reason parents avoid assisting in hatching except sometimes near the end of the process when the membranes are already breached (Brua 2002) (see discussion of a likely occurrence of this at one of the nests on cam);
    • Accidental damage by a parent or sibling before hatch is complete;
    • A “capped shell,” where a large part of shell from a previously hatched egg slips over the large end of the hatching egg, forming a double layer of shell that the chick might not be able to break through.
Infertile or nonviable?

Even under laboratory examination (which is rarely done with Bald Eagle eggs), it is often impossible to know whether a broken or unhatched egg was infertile or nonviable. (Birkhead et al. 2008) If an embryo stopped developing in the first few days after the egg was laid when it was still only a few cells in size, it might not be detectable by candling or even under a microscope. (Cooper 1993; Houston et al. 1993; Fox 1995; Birkhead et al. 2008; Hemmings et al. 2012) Conversely, if an embryo’s death occurs late in the gestation period, its body fills the shell and nonviability is obvious.

An infertile egg dries up and becomes fragile, often breaking up eventually, although many times it remains intact throughout the incubation period. Likewise, a nonviable egg may remain intact, or it may break apart or burst open, depending on the cause and the timing during the incubation period. The death of an embryo brings to an end the organic processes of defense against bacteria, which then proliferate and contaminate the egg’s contents (Houston et al. 1993), further hampering a laboratory examination.

If the egg does not break apart, the parents do not know whether it is infertile or nonviable, and they may continue to incubate it for days or even weeks beyond the time it should have hatched. Continued incubation is less likely if there is a hatchling in the nest, since the egg eventually will get in the way of the growing eaglet, the nest cup will become less deep, and there will not be room for a parent to incubate. Eventually an unhatched egg may be buried or trampled into the nest or even partially consumed by adults or fed to a hatched nestling. The parents may move pieces of shell out of the nest cup. Unhatched eggs may remain in the nest for months after the nest is vacated; they may be destroyed by predators like crows, ravens, or raccoons.

SECOND CLUTCHES

AT WILD BALD EAGLE NESTS

© elfruler 2020, 2024

See full citations of References mentioned here.

Many species of birds routinely lay more than 1 clutch of eggs in a breeding season and raise the eaglets to fledge and coach them through the juvenile training period. Large birds like Bald Eagles normally do not lay a second clutch for the simple reason that their breeding season is not long enough. The incubation period for eagle eggs is more than 5 weeks long, and after hatching the nestlings require 10 or more weeks to grow and fully develop before they fledge. They then must train in flying and foraging for food, which can take 5 months or more.

But sometimes after losing a clutch an adult eagle pair will lay a second, replacement, or “double” clutch. A few instances have been described in scholarly literature (see below). At the Bald Eagle nests observed via video cam from 2006-2020 (here is a list of nests included in the data), a total of 389 first clutches of eggs were laid, of which 44 failed (11.3%) (see this Table). Of the 44 failed clutches, the adults at 12 laid second clutches, which is 27.3% of the failed first clutches, or only 3.1% of total clutches. Table A enumerates these second clutches and gives the cause of the loss of the first clutch if known, dates and time intervals, and the ultimate outcomes.

TABLE A

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Of the 12 second clutches at these nests, only 2 ended successfully (16.7%): at the Pittsburgh Hays nest in 2017, with 1 fledge, and at the Southwest Florida nest in 2020, with 2 fledges.

There is some uncertainty about whether the events at the Pittsburgh nest in 2017 fall in the category of a failed first clutch followed by a second clutch. The nest tree fell 2 days after the first egg was laid, during which the female may have been carrying a second egg, but this was not observed from the ground. The adults miraculously built a new nest in which another egg appeared 7 days after the loss of the first egg. The time interval and especially the building of a new nest point to a second clutch.

The events at the Southwest Florida nest in 2020 are unique among the nests surveyed, in that the second clutch came after the loss of the only eaglet from the first clutch, rather than after the loss of eggs. Production of a second brood of eaglets is a rare occurrence among Bald Eagles. See below for further discussion.

What determines whether the adult pair lays a second clutch?

The most important factor is timing. Once a clutch of eggs is complete and the adults begin incubation, their hormonal reproductive cycles begin to progress to a new phase. The female’s ovarian follicles stop producing ova, the male gradually produces fewer sperm, and the changing hormonal balance induces incubation behavior.  In order to lay a new clutch, the hormones must “recycle” back to the beginning of the egg-laying cycle. (See Reproduction & Hormones page.)

A second clutch will occur only after the loss of all eggs of the first clutch. The point after which hormonal recycling is unlikely at most nests (except in southern regions) appears to be about halfway through the incubation period, when the hormones prepare the adult’s body to begin its annual feather molt (see Fox 1995; Heidenreich 1997; Winkler 2016). The average incubation time for a Bald Eagle egg is about 36-37 days (see stats here), or for both eggs in a 2-egg clutch, 36-40 days; so the halfway point in incubation would be about 18-20 days. Nests in Florida, Louisiana, and other states in the Sub-tropics may be able to recycle their hormones somewhat later in the incubation period. (See discussion of reproduction timing here.)

Among the nests with a second clutch in Table A (excluding the unusual second brood at the Southwest Florida nest in 2020), the time interval from the first egg to loss of the first clutch (highlighted in blue in Table A) ranged from a few seconds (CA Sauces Canyon 2020) to 19 days (IA Decorah North 2018), with an average of about 8.7 days. This supports the idea that hormonal recycling in the female is unlikely after about the halfway point in incubation of the first clutch.

By way of comparison, Table B lists the 33 nests with a failed clutch that did not have a second clutch (as far as is known).

TABLE B

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Most of the clutches at these nests were lost in the second half of the incubation period. The 4 exceptions were:

    • ME Hancock County 2011 (6 days); possible weather disturbance, eggs abandoned
    • CA Sauces Canyon 2013 (11 days); intruder, female disappeared, eggs abandoned
    • MD Blackwater 2016 (~2 weeks); possible intruder, nest abandoned
    • WV Shepherdstown 2018 (~19 days); intruder, female disappeared

In 3 of these instances intruders likely deterred the resident pair from reclutching, while the 4th, the Maine nest, is probably too far north for the pair to have had time for a successful second clutch.

These cases indicate factors that can affect whether hormonal recycling leading to a second clutch occurs:

    • LOCAL CLIMATIC CONDITIONS. This affects several factors, including length of the breeding season, availability of adequate food to nurture growing eaglets, and the possibility of challenges during incubation and rearing, such as high heat, high humidity, bad weather, and proliferation of ectoparasites.
      • Research indicates that southern nests are more likely to see second clutches because of a longer season during which eaglets can grow, fledge, and learn survival skills in the wild. Hensel & Troyer 1964 point out that Bald Eagles in Alaska and Canada are unlikely to lay a second time. In Florida and more southerly regions, second clutches are more common.
        • Among the 12 nests with second clutches in Table A, the 9 in VA, AZ, FL, and the Channel Islands have relatively mild climates and longer seasons, making a reclutch feasible.
        • On the other hand, in the other 3 nests in Table A the climates can be more unpredictable, and the breeding seasons are shorter. At the IA Davenport and PA Pittsburgh nests the reclutching began 11 and 7 days after the loss of the first clutch, respectively, while the IA Decorah North pair took 27 days to recycle. The Decorah hatchlings were plagued by excessive heat in May and a proliferation of black flies which led to their deaths, perhaps a cautionary tale.
        • Among the 33 nests that did not see a second clutch, listed in Table B, those in BC, OR, ME, MN, and WI nests are in northern latitudes with shorter breeding seasons, likely precluding a second clutch.
      • THE TIMING OF EGG-LAYING WITHIN THE BREEDING SEASON can affect whether a second clutch is laid and produces fledglings. Birds time their breeding efforts to coincide with the optimal time for adequate food resources to nurture growing eaglets and young fledglings. (See discussion of timing of Life History events here.) Some research suggests that earlier clutches are more successful than later ones. (Blanco et al., 2007)
        • All of the nests in Table A laid their first clutches early in the season, allowing sufficient time for a second clutch.
        • At the nests in Table A, the time interval from loss of the first clutch to the beginning of the second ranged from 7-28 days (highlighted in orange), with an average of about 20 days (not counting the second brood of eaglets at Southwest Florida).
        • Of the nests that did not lay a second clutch, listed in Table B, most generally lost their first clutches later in the season.
      • THE CAUSE OF THE FIRST CLUTCH’S FAILURE. If intruders, predators, human disturbance, bad weather, or other uncontrollable external events brought about the egg loss, the adults may not be moved to repeat the risk, especially if the disturbances continue.
        • External events like bad weather and human disturbance can disrupt the food supply, which forces the eagles to weigh whether there would be enough resources to care for eaglets while also maintaining their own health. (See Morrison & Walton 1980; Evans & Heiser 2004)
        • Extreme temperatures can affect semen production, ovulation and ovum development, timing and effectiveness of copulation, and fertilization.
        • A fallen nest is a strong deterrent to laying a second clutch because of the cost of building a new nest. This is not unknown, though, as happened at CA Redding/Turtle Bay in 2017 and PA Pittsburgh Hays in 2017 (in an astounding 1 week!).
          • Even if a fallen nest is not the cause of a clutch failure, eagles may build a new nest for a replacement clutch. Simons et al. 1988) report that of 33 females in Florida who laid second clutches over 3 years (1985-1987), 12 relaid in different nests from the original ones. In some of the cases listed in Table A, it is possible that the eagles did lay a second clutch in a second nest that was not visible from the nest cam.
Published reports of second clutches

A few accounts of second clutches appear in published reports. The earlier reports lack specific details, especially of dates, time intervals between events, and number of eggs. Until recent years with the installation of nest cams, very few Bald Eagle nests in the wild have been observed closely enough to know the timing of the loss of a first clutch and the laying of a second.

    • Herrick 1934 observed a handful of second clutch instances at nests in FL, but he does not mention time intervals from first to second clutch.
    • Bent 1937 tells of one FL nest where the adults laid a replacement clutch after “about two months.”
    • Fox 1995 asserts that for raptors (not Bald Eagles specifically) it can take 2-3 weeks after clutch loss before the female is able to lay again.

Most reports of second clutches come from descriptions of captive breeding programs or restoration projects:

    • Wiemeyer 1981 describes the captive breeding program at the Patuxent Wildlife Research Center in MD from 1976-1980, where the first clutch eggs were removed from 11 nests about 5-8 days after the clutch was completed. The adults at 9 of the nests laid second clutches, from 18-23 days after the first clutch was removed. 4 of the second clutches were successful.
    • Heidenreich 1997 reports that the eggs of the first clutches of 9 captive Bald Eagle pairs were removed 2-3 days after the last egg was laid, and a second clutch came from 22-57 days later, an average of 32 days from first clutch loss to the second clutch.
    • Simons et al. 1988 and Wood & Collopy 1993 describe the undertaking of the Sutton Avian Research Center in OK and the Florida Game and Fresh Water Fish Commission from 1985-1988 to remove clutches of eggs from wild Bald Eagle nests in FL, artificially incubate them at the Center, and raise the eaglets to fledge from hack towers. Over the 4 seasons they removed 124 eggs from first clutches at 58 nests when the eggs were about 16 days old. Adult pairs at 45 of the nests laid second clutches (77.6%), 14 of them in different nests than the originals. The time interval from egg removal to the second clutch ranged from 20-57 days, or an average of 29.4 days. From 1984-1987, 66.7% of the second-clutch nests produced fledglings. (Of the 87 eggs removed from first clutches and incubated at Sutton from 1984-87, 59 of them, or 68%, resulted in hacked fledglings.)
    • Sharpe & Garcelon 2003 report on efforts of the Bald Eagle Restoration program on the CA Channel Islands undertaken by the Institute for Wildlife Studies, including repopulating the islands with young eagles from northern CA, WA, and BC, monitoring breeding activities, collecting unhatched eggs and analyzing them, gathering newly laid eggs for artificial incubation, and fostering chicks back into nests to be reared to fledge. Aside from the instances of second clutches observed on cam included in Table A (Sauces Canyon in 2014, 2017, and 2020, and West End in 2020), Sharpe et al. 1998, 1999, and 2018, researchers encountered several instances of replacement clutches at some nests without cams:
      • At the nest on Pinnacle rock on Catalina Island in 1998, 1 egg was removed for artificial incubation on March 25, 2 days after it was laid. It was replaced with an artificial egg, but the eagles did not accept it and built a new nest a few hundred meters away. Within 1 day, by March 26, they started a second clutch with 1 egg in the new nest, but that egg was gone by the next day, and the eagles disappeared. 28 days later, on April 23, they were incubating a new egg in the new nest, their third clutch. This egg was removed on March 14 and replaced with a dummy, which the eagles incubated. On May 10 a chick from a different nest (West End) was fostered into the nest and it fledged on July 22.
      • The West End (Catalina Island) nest in 1999 was occupied by 1 male and 2 females, and the male copulated with both females. Two eggs being incubated from March 6 were removed and replaced with artificial eggs 2 days later on March 8. On March 13, 5 days after the eggs were removed, a third egg was seen, which may have been part of the original clutch and unnoticed by researchers on March 8, or it may have been laid by either female as a second clutch.
      • The eagles in the Seals Rocks nest on Catalina Island in 2018 began incubating 1 egg on February 16 but it was lost 3 days later on February 19. They had begun a second clutch with 2 eggs by April 10, 50 days after loss of the first clutch. One chick hatched and fledged by July 30.

Researchers who conducted these programs found that second clutches were more likely if the first clutch eggs were removed during the first half of the incubation period.

EGG PULLING is the practice of removing each egg from a nest immediately after it is laid, even before a clutch is complete, and before adults begin incubating. The removal often results in continued egg-laying as long as the female detects no egg in the nest. Gilbert et al. 1981 report on such an effort at the National Zoological Park in Washington, DC in 1979, where “each egg was removed on the day of laying which, incidentally, resulted in the female laying seven eggs in rapid succession.”

At the CA Sauces Canyon nest in 2017, each of the unprecedented 5 eggs in the first clutch broke shortly after being laid. After each egg broke, three days later the female laid a new egg, stopping after 5 eggs over a 12-day span. This seems comparable to the effects of egg-pulling except for the human factor.

Heidenreich 1997 points out that egg pulling can endanger the health of the female unless she is provided with a diet that replaces nutrients depleted by continual production of ova, yolks, albumen, and eggshells.

Second broods of eaglets

The unusual second brood at the Southwest FL nest in 2020 came after the 37-day-old eaglet of the first clutch died, 64 days after the first egg was laid on 11/12/19. Hormonal recycling and beginning of the second clutch (2/22/20) came 38 days after loss of the first brood on 12/19/19. As noted earlier, the second clutch had a perfect outcome, with both eaglets fledged.

Reclutching by Bald Eagles after loss of an eaglet is quite rare, but I am aware of published reports of 3 such occurrences, all in southern nests, and all resulted in fledges:

    • Shea et al. 1979 observed an incubating adult by aerial survey in Everglades National Park in southern Florida on 11/22/74. Photographs taken from the ground in the first week of January showed 2 nestlings about 7-10 days old in the nest. But an aerial survey on 1/8/75 revealed an empty nest with 2 adults perched beside it. On 2/27/75 the researchers saw 2 eggs in the nest and then 2 hatchlings about 3/20/75. Both eaglets of this second brood fledged around 6/15/75.
    • Bryan et al. 2005 observed eagles in south central South Carolina beginning a breeding effort in November 1998 and saw them feeding a nestling on 12/9/1998, suggesting egg-laying around 11/1/98. About 7 days later the adults abandoned the nest “for unknown reasons,” a loss perhaps 45 days after the egg was laid. They laid a second clutch in late February 1999 (about 10 weeks after losing the first brood), which produced 2 nestlings. The eaglets were found on the ground in May and June and were rehabbed at the South Carolina Center for Birds of Prey, from which they were released in August.
    • Krol 2018 reports that in fall 2016 a pair of Bald Eagles built a nest in a cove on Jordan Lake, North Carolina, after having nested the previous year on the other side of the cove. By 12/6/16 the pair were incubating, and the author saw a 1-week-old nestling being fed on 1/18/17. He saw the 4-week-old nestling again on 2/9/17, but by 2/16/17 the nest had partially collapsed and was empty, and the adults were not in sight. The next day an adult pair were observed at the nest that had been in use the year before. This pair were incubating by 4/5/17, and parental behavior on 4/12/17 suggested the presence of a hatchling, from an egg which the author estimates was laid about 3/6/17. The author saw feeding occur on 4/16/17, and he saw 2 nestlings from late April through early May, but only 1 on 5/5/17. This eaglet fledged around 7/18/17. Although the author did not directly observe the adults from the failed nest move to the other nest, his argument that it was the same pair at both nests is compelling.

In these 4 cases (counting Southwest FL), the time from the first egg to the loss of the brood ranged from about 46-72 days. The incubation period had long ended and the parents had begun responding to a changed hormonal balance that induced behaviors of nurturing and feeding their growing nestlings. After the loss, the hormonal recycling for laying a second clutch took from about 19-70 days. Table C gives the time intervals for the 4 nests.

TABLE C

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The first clutches all were laid early in the breeding season, from early November to early December, and the losses were early enough to allow adequate time for a successful second brood to fledge and undergo training. The wide range of timings between the first egg and the loss of the brood and between the loss and the beginning of the second clutch reveals no trend that could predict the probability of a second brood of eaglets after loss of the first. But it does illustrate the likelihood that only in southern regions is such an occurrence likely. It also showcases the remarkable ability of Bald Eagles to adapt to achieve reproductive success, even if the eagles themselves are not consciously doing so.

LOST NESTLINGS AND FAILED BROODS OF EAGLETS

AT WILD BALD EAGLE NESTS,
2006-2020

© elfruler 2020

Lost Nestlings

20.8% of the eggs laid at the observed nests from 2006-2020 were lost. (See discussion here.) But the number of nestlings lost before they could fledge was fewer, 16.2%. As a percentage of the number of eggs laid, the number of nestlings lost was 12.9%.

Table 5

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Losses of nestlings were roughly equivalent across clutch size:

    • 1-egg nests lost 16.7% of their nestlings.
    • 2-egg nests lost 16.9% of their nestlings.
    • 3-egg nests lost 15.2% of their nestlings.
    • 4-egg nests lost 22.2% of their nestlings.

This contrasts with the more dramatic differences among clutch sizes in the loss of eggs, where 1-egg nests were far less successful with 55.6% losses, and 3-egg nests were significantly more successful with only 16.7% losses of eggs.

Causes of nestling loss, as with egg loss, include external events, such as bad weather, a fallen nest, Bald Eagle intruders, and intrusions by other animals. But nestling losses also come about for reasons that don’t apply to eggs, including fall from the nest, injury, starvation, ectoparasites, disease, and poisoning. As with egg losses, many causes are observable on cam, but often the cause cannot be perceived from afar. If a nestling’s body can be retrieved from the nest without disturbing the other eagles, laboratory analysis might reveal a cause, but sometimes even then the reason is elusive.

The highest percentage of lost eggs were brought about by intruders (see Table 3), but it was bad weather that caused the most lost nestlings. This is no doubt due to the likelihood that nestlings are often exposed to the elements, whereas eggs remain more protected throughout the incubation period.

    • 19.7% nestlings were lost because of bad weather.
    • 13.7% fell from the nest.
    • 4.3% were predated.
    • 4.3% starved.
    • 4.3% were lost because of intruders.
    • 3.4% were injured.
    • 2.6% were victims of ectoparasites.
    • 1.7% of losses were due each to disease and poison.
    • The causes of a large plurality of losses, 44.4%, were unknown.
Failed Broods of Nestlings

There were 350 broods of nestlings at the nests from 2006-2020, and 8.9% lost all of their eaglets. Again, causes of some of the failed broods are known, but many are not. Table 6 enumerates the failed broods at specific nests (referred to by abbreviated codes, which are identified at the end of the table) and gives the cause, if known.

Table 6

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As with losses of clutches of eggs, 1-egg nests had the highest rate of failed broods of nestlings:

    • 1-egg nests lost 16.7% of their broods.
    • 2-egg nests lost 8.9% of their broods.
    • 3-egg nests lost 7.4%. of their broods
    • 4-egg nests lost none of their broods.

The number of broods of nestlings lost was highest at 4 in 2012, 2017, and 2018. But 2012 lost the highest percentage of total broods, with 17.4% lost. 2006 and 2008 had no failed broods, and only 1 brood failed in 2011, 2013, and 2019. The percentage of losses in 2019 was quite low, with only 2.9% lost.

Note that the second brood of eaglets at the Southwest Florida nest is included in the total number of broods. It is the only such second brood of nestlings in the data. (See discussion here.)

SUCCESS RATES OF CLUTCHES AND BROODS

AT WILD BALD EAGLE NESTS,
2006-2020

© elfruler 2020

Table 2 drills down more deeply into the clutches of eggs and broods of eaglets at the nests observed, showing the number of clutches of each nest size (1 egg, 2 eggs, etc.) each year, the number of clutches with hatched eggs in each nest size, and the number of broods with fledged eaglets in each nest size. The first page of the table gives numbers for clutches of eggs, which includes second clutches. The second page gives numbers for broods of eaglets and fledges.

The figures in the table refer to the number of clutches or broods of a particular size (1-hatch or 1-fledge clutches, 2-egg or 2-fledge clutches, etc.), not to numbers of individual eggs, chicks, or fledges, which are tallied in Table 1. Percentages illustrate the degree of success of a clutch or brood.

I use the term “successful” in reference to a clutch in which at least one egg hatched and to a brood in which at least one egg hatched and at least one eaglet fledged. An unsuccessful clutch is one in which no eggs hatched, and an unsuccessful brood is one in which no eaglets fledged.

I use the term “perfect” in reference to a clutch in which all eggs hatched and to a brood in which all eggs hatched and all eaglets fledged. Perfect clutches and broods are highlighted in orange.

TABLE 2

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Clutches of eggs with hatches (p. 1 of the Table)

Of 401 clutches of eggs, 87.5% were successful and 66.3% were perfect.

    • 1-egg clutches averaged a 44.4% success rate, which of course is the same percentage for perfect clutches, since only 1 egg is involved.
    • 2-egg clutches averaged a much higher success rate of 87.7%, with 70.1% perfect with 2 eggs hatched.
    • 3-egg clutches averaged an eye-popping 96% success rate, with 63.5% perfect with 3 eggs hatched.
    • 4-egg clutches hit the jackpot with a 100% success rate, 2 out of 3 of which (66.7%) were perfect with all 4 eggs hatched.

The low success rate of 1-egg clutches can be attributed at least partially to the fact that if the only egg is lost, the clutch is lost. The same could apply to the higher rate of success of both 2-egg and 3-egg clutches, with more eggs to “spare.” But the high rate of perfect 2-egg and 3-egg clutches defies this logic and perhaps points to subtle behavioral or biological factors such as parental attentiveness or the reproductive superiority of adults who succeed in laying more eggs than one.

Broods of eaglets with fledges (p. 2 of the Table)

Of the 401 clutches of eggs in Table 2, 76.8% ended up with successful broods of fledged eaglets, and 46.9% resulted in perfect broods.

    • 1-egg clutches averaged 33.3% successful and 33.3% perfect rates of fledged eaglets.
    • 2-egg clutches averaged 77% successful, with 50.8% perfect with 2 eaglets fledged.
    • 3-egg clutches averaged 85.7% successful, with 42.9% perfect with 3 eaglets fledged.
    • 4-egg clutches were 100% successful in producing fledglings, but only 1 out of 3 clutches, or 33.3%, resulted in a perfect 4 fledged eaglets.

Comparing the success rates of broods of eaglets with success rates of clutches of eggs illustrates well the challenges that hatched nestlings and their parents face in achieving the full development and growth from hatch to fledge. In all except 4-egg clutches, the percentage of successful broods dropped by a little over 10 points from the percentage of successful clutches.

    • 1-egg nests had 44.4% successful clutches but only 33.3% successful broods.
    • 2-egg nests had 87.7% successful clutches but only 77.0% successful broods.
    • 3-egg nests had 96% successful clutches but only 85.7 successful broods.
    • 4-egg nests had a 100% successful rate for both clutches and broods.

As noted in the discussion of Table 1, some fledges could not be confirmed. In Table 2 where the numbers refer to clutches and broods rather than to individual eggs or eaglets, a nest where at least 1 eaglet’s fledge is not confirmed is counted in the unconfirmed row, even if at least 1 eaglet did fledge.

Table 2, like Table 1, shows that numbers can fluctuate up and down from one year to the next, and there is no clear trend in either direction.  For example:

    • Successful clutches hit a peak of 100% in 2007, and a low of 75%in 2015. 2019 was above average with 89.7% successful, but 2020 was below average with 76.7%.
    • Perfect clutches ranged from a low of 50% in 2006 to an astounding 91.3% in 2012. 2019 was slightly above average at 66.7%, while 2020 was well below average at 53.5%
    • Successful broods were at a low 50% in 2006, with a high of 87% in 2011. 2019 was well above average with 84.6% successful, while 2020 fell slightly below average with 72.1%
    • Perfect broods were low in 2006 with 33.3%, but very healthy in 2010 at 63.2%. 2019 had an above average rate of 48.7% perfect broods, and 2020 was at the lower end of the range with 39.5%.

EGGS, NESTLINGS, AND FLEDGLINGS

AT WILD BALD EAGLE NESTS,
2006-2020

© elfruler 2020

The table presented on this page gives numbers of eggs, nestlings, and fledglings observed at wild Bald Eagle nests on streaming video cams or by credible ground observers from 2006-2020. (Click here for a list of nests providing data.)

Table 1 gives precise counts of eggs laid, chicks hatched, and eaglets fledged over the 15 breeding seasons, broken down by year and by clutch size (1-egg, 2-egg, 3-egg, 4-egg, 5-egg), with totals and percentages.

TABLE 1

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Here are some highlights:
    • The data incorporate numbers from a total of 388 nesting seasons, resulting in 401 clutches of eggs including 12 second clutches after loss of the first clutch (see Notes at the bottom of the table).
    • Of the total clutches for the 15-year time frame,
      • 1-egg clutches made up 6.7% of total clutches;
      • 2-egg clutches made up 60.8% of total clutches;
      • 3-egg clutches made up 31.4% of total clutches;
      • 4-egg clutches made up 0.7% of total clutches.
    • A total of 910 eggs were laid.
    • The average number of eggs laid per clutch was 2.3.
    • 721 of the eggs hatched, or 79.2% of the eggs laid.
    • Of the clutches in which at least one egg hatched,
      • in 1-egg clutches, 44.4% of the eggs hatched;
      • in 2-egg clutches, 78.9% of the eggs hatched;
      • in 3-egg clutches, 83.3% of the eggs hatched;
      • in 4-egg clutches, 75% of the eggs hatched.
    • A total of 588 eaglets were confirmed to have fledged, either directly from the nest (571), or after rehab and release (17). This is 81.6% of nestlings hatched, or 64.7% of eggs laid.

In a handful of cases it is unknown whether a particular eaglet fledged, sometimes when the cam went down, or the cam angle made it impossible to follow an eaglet’s movements, or an eaglet had a misstep and fledged before it seemed ready and ground searches could not confirm it was safe.

Numbers can fluctuate up and down from one year to the next, and there is no clear trend in either direction.  For example:
    • 14.6% of the clutches in 2018 had 1 egg, none in 2019, and only 7% in 2020. The numbers of 3-egg nests jumped from 28.6% in 2013 to 41.4% in 2014, then dropped to 21.4% in 2015.
    • The total number of eggs hatched ranges from 66.7% in 2006 to 96.3% in 2012.
    • Sheer numbers can be deceptive. In 2020 a whopping 29 eggs were lost, but that is 70.7% of the total number of eggs laid that year, slightly below the average of 79.2% for all eggs hatched.
    • Confirmed fledges (directly from the nest and rescue/rehab) varies between 69.2% in 2012 and 93.5% the year before, 2011.

The fluctuations in the numbers actually reflect what observers have seen happen on the nests. Many factors affect the success of a given nest in a given year. These include weather, change of nest, change of mate, food availability, intruders and predators, and unusual events such poisoning and accidents. The variations also could be reflective of the relatively small sampling of nests.

NUMBERS FROM THE NESTS

WILD BALD EAGLES, 2006-2020

© elfruler 2020

The video cameras that have been trained on Bald Eagles’ nests since 2006 have provided a treasure trove of information about the breeding behavior of these apex raptors. In the universe of the more than 100,000 active Bald Eagle nests in North America, the data that these particular nests yield is minuscule. A few published scholarly reports on Bald Eagle nesting success focus mainly on a circumscribed area (e.g. Florida) for 1 or a few breeding seasons. The data here from the nests on cam span 15 years of breeding from 2006-2020 across a wide geographical expanse throughout the continent, and they represent the full range of climates and habits in which Bald Eagles reproduce. (Nests included in the data are listed here.)

Over the period, adult pairs at these nests made 401 breeding efforts at 85 locations, producing 910 eggs, 721 hatchlings, and at least 588 fledglings. These numbers might be considered a fair sampling of breeding data for the species.

The pages and tables that follow break down the data collected via these cameras on multiple levels. The raw numbers of eggs laid, nestlings hatched, and juveniles fledged, from nest to nest and year to year, yield statistics and percentages that give an overall view of breeding success over the 15 years. Burrowing more deeply into these numbers reveals how many clutches are successful over time, and which clutches of a particular size (1 egg, 2 eggs, etc.) are more successful than others. The numbers open a window into losses of eggs and eaglets, and what we can learn about reasons for those losses. And the numbers help flesh out some perceptions of behaviors of nesting Bald Eagles, such as coping with bad weather, predators, and intraspecific intruders (by other Bald Eagles), and replacing a lost clutch.

The data reinforce some facts that are already known:  Bald Eagles typically lay clutches of 2 eggs, with clutches of 3 eggs less common, clutches of 1 egg unusual, and clutches of 4 eggs quite rare. A fair number of eggs do not hatch, but a healthy majority end in successful fledges.

Other details to emerge from these analyses are perhaps more surprising:  While overall averages seem consistent with what is generally believed, there is often a wide range of values across seasons and from nest to nest.  In some years the number of eggs lost far exceeds the average, while in other years few eggs remain unhatched. Similarly, the number of nestlings that die before fledging covers a wide range among the years. Three-egg nests produce a higher percentage of fledges than either 2-egg nests or 1-egg nests; the latter are least successful in producing fledges.

These pages represent a complete revision of data that I published here in 2018, which consisted of a single page and 1 spreadsheet. For this new report I have pared down the nests to include only those with the most reliable observations, mainly the ones with streaming video cams, plus a small number of nests with reliable ground observers. I have also expanded the detail and breadth of information and analysis, resulting in 8 spreadsheets, and I have provided a narrative discussing each one. I have also compiled a lengthy list of References to literature on breeding, eggs, incubation, and survival.

These tables and narratives are presented in sequence in the pages that follow:
Additional new pages also make use of the nest data:
Full references for citations in the following pages are given here:

I began collecting data when I started watching web cams in 2009. Thanks to the Hancock Wildlife Foundation, the Institute for Wildlife Studies, spreadsheets compiled by Judy Barrows, nest cam websites and Facebook pages, and numerous individuals with whom I have communicated, I have been able to stretch the data back to 2006 when streaming cams first began operating. These sources also have been invaluable in filling in gaps in my own observations. I owe all of them a great debt of thanks.

 

INTRASPECIFIC INTRUSIONS AT BALD EAGLE NESTS

© elfruler 2018

“intraspecific adj. : occurring within a species or involving members of one species.” (www.merriam-webster.com)

Bald Eagles choose their breeding territories and nest sites carefully, driven by factors that will lead to success in raising their young.  These factors include adequate food resources, a sturdy nest platform, available shelter from dangerous weather, ease of defense, and tolerable distance from disturbances.  A good location will be attractive to any Bald Eagles that come along, and it is not surprising that a resident Bald Eagle pair will be challenged by other Bald Eagles for the site, leading to competition between members of the species, or intraspecific conflict.

It is not uncommon for one or both members of a pair to be challenged even before the nesting season begins, resulting in displacement, injury, and even death.  Conflicts that occur once a clutch of eggs has been laid or a brood of chicks has hatched can cause loss of eggs and chicks, despite the fierce defense that the parents inevitably mount against intruders.  Often the parents are successful in repelling a challenge and their chicks fledge.  In rare cases an intruder is accepted by the resident pair as a cooperative breeder (Go here for a series of pages about Cooperative Breeding and its occurrence among Bald Eagles).

The Bald Eagle nests that have been viewed on camera or monitored carefully from the ground since 1992 have provided a glimpse of intraspecific intrusions of many types and with a variety of outcomes:

  • Events before, during, and after the season
  • From one to many intruders
  • Intruding males and females, adults, subadults, and juveniles
  • Replacements, disappearances, injuries, and deaths of parents
  • Unhatched or broken eggs and injured or slain nestlings
  • Double clutches
  • Rescued nestlings and rehabbed fledglings
  • Successful fledges
  • Cooperative breeding

Even careful monitoring of cams and nests don’t provide the full picture of events surrounding intrusions, which often take place out of human view.  Even happenings in full view do not always have clear-cut explanations.

  • How many intruders are in the area?
  • When did they first appear?
  • What encounters occur off-nest between intruders and residents?
  • What is the sex or age of an intruder?
  • Does a resident eagle disappear because it has been injured or killed, or because it has decided that it cannot prevail in a battle?
  • Was an egg broken by an intruder or an agitated parent, or because it was unviable or infertile?
  • Did an egg fail to hatch because intruders interrupted the reproductive cycle and prevented fertilization?
  • Why would one intruder destroy eggs or chicks but another intruder leave eggs or chicks undamaged?
  • Does a resident adult respond primarily defensively to an intruder, or might there be a trigger that precipitates an offensive response?
  • Why would an intruder become a helper and cooperative breeder rather than a threat?

Answers to such questions would provide a much better understanding of events we can see, but too often the answers elude us.

The number of Bald Eagle nests for which reliable daily reports have been made more than doubled from 2008 to 2018 – from 24 to 57.  They range from southern California to New England, from south Florida to Alaska, and many points on the continent inbetween.  The nests are found in a variety of habitats, including rural farmland, along rivers and streams, lakes and bays, in woodlands in parks and wildlife refuges, on coastal islands, college campuses, and in city neighborhoods.  Yet it cannot be claimed that these nests are a representative sampling of all of the thousands of Bald Eagle nests in North America.

Nor can we be sure that we have witnessed every conceivable behavior or outcome associated with intraspecific intrusions.  The information that I present here is illustrative of certain types of Bald Eagle behavior, but should not be taken as a statistical report on intraspecific intrusions.

This first chart (at this link) describes the intraspecific intrusions observed at Bald Eagle nests from 1992 through 2018, the nest locations and habitats, what is known about the intruders, the events, and the outcomes.  (Link opens in a new browser tab.)  Losses (of parents, eggs, or chicks) can result directly or indirectly from intrusions, or they may occur for other reasons not related to intraspecific conflict.  Not every intrusion leads to a loss.

This second chart (follow this link) summarizes the details and gives percentages to enable comparison of intrusions, losses, and fledges from one year to the next.  (Link opens in a new browser tab.)  The percentages are number of intrusion events (nest intrusions, clutch intrusions, eggs lost, etc.) compared to the total number observed (nests, clutches, eggs., etc.) in that season.  (The years 1992-2007 at the California West End nest are not included in this summary because of the outsized effect of DDE contamination on egg production there.)  Notably, for these nests there is no clear trend in the percentages over time.

  • While 2018 clearly was a bad year for nest intrusions (including both before and during a clutch), at 24.6% of observed nests, 2008 was almost as bad, at 20.8% of observed nests.
  • On the other hand, 2008 was a worse year for clutch intrusions (after eggs were laid), at 19.2% compared to 17.9% in 2018.
  • And in 2008, 4.5% of chicks were lost, compared to a much smaller 1.3% in 2018.
  • The number of eggs lost was a staggering 11.3% in 2018, but the 8.6% of eggs lost in 2008 is the second highest percentage.
  • The year 2013 was difficult, with 13.6% of clutch intrusions and 6.7% of egg losses.
  • Some years were relatively benign: 2011 saw only one intrusion and 2012 only two.  A dip in losses occurred in 2015 and in intrusions in 2016.
  • From 2013-2017 most percentages were relatively stable – nest intrusions, clutch intrusions, losses – with an overall dip in 2016. Intrusions and losses in 2018 were severe, and it remains to be seen in coming seasons whether that year was an outlier.
BRIEF UPDATE ON THE 2018-2019 SEASON: I have not yet added new information to the charts, but the number of intraspecific intrusions declined significantly from the year before. Only 6 such intrusions occurred, only 2 of which happened after eggs were laid. 2 nests ended up with no eggs laid, and a total of 4 eggs were lost. No eaglets were lost.

There is no question that suitable habitat for nesting Bald Eagles is on the decline across the continent because of human development and encroachment.  But the numbers we have for these particular nests do not necessarily mean that increasing numbers of nest intrusions point to an approaching saturation of carrying capacity for Bald Eagles across the board.  Each territory has its own conditions that may or may not be either conducive or resistant to nest intrusions.  Increasing population density in a particular area may simply drive some Bald Eagles to adapt by seeking out previously unclaimed territories, by gradually shrinking the size of their territories (over time) to allow for more nests (if the food supply allows for it), or by allowing more instances of cooperative breeding.  It remains to be seen whether intraspecific intrusions will have a negative impact on the Bald Eagle population in the long run.  Some have argued that a rise in population density ultimately could result in a state of population equilibrium by slowing the breeding productivity to offset the long period of increase that followed the banning of DDT in 1972.

While the snapshots that these observed nests provide give us some narratives about intraspecific intrusions at Bald Eagle nests and make comparisons possible, a broader understanding of the causes and effects of such intrusions, as well as a glimpse of what they may entail in the future, must await more detailed and systematic studies (such as Mougeot et al. 2013 in Saskatchewan and Turrin and Watts in the Chesapeake Bay, 2014 and 2015).

For perspective on the Bald Eagle population in North America and trends over time, Partners in Flight (PIF) estimates the number of breeding-aged Bald Eagle individuals in 2017 at around 250,000, based on data from the North America Breeding Bird Survey, an approximate 131% increase since 1970.  The U.S. Fish & Wildlife Service’s oft-cited number of about 10,000 breeding pairs (or 20,000 individuals) in the lower 48 United States in 2007 does not include numbers from Canada or Alaska (both of which exceed the number in the lower 48 states), and it represents only eagles in pairs that are actively breeding.  The PIF estimate encompasses all individual Bald Eagles throughout North America of breeding age whether they have formed breeding pairs or not.  None of these numbers include juvenile or subadult Bald Eagles, which could more than double the totals.

 There is as yet no sign that the Bald Eagle population is declining, whether because of habitat changes that lead to overpopulation and intraspecific conflict in a territory, or other causes such as contaminants, trauma, electrocution, disease, poisoning, and poaching.  In 2010, following the removal of the Bald Eagle from the list of threatened and endangered species, the U.S. Fish & Wildlife Service produced a Post-delisting Monitoring Plan for the Bald Eagle.  The Plan establishes a 20-year monitoring period (roughly four generations of breeding Bald Eagles) in the lower 48 states, with data analyzed and reported to the public every 5 years.  The Plan will yield information on changes in numbers and their causes, and it includes provisions for responding to a 25% or greater decline with corrective action by federal, state, and local agencies, Native American Tribes, and other interested partners.  The Plan specifically references the possibility of re-listing the Bald Eagle as threatened and/or endangered as a remedy to an unacceptable level of decline.

REFERENCES

Dzus, E.H. and J.M. Gerrard 1993.  Factors influencing Bald Eagle densities in northcentral Saskatchewan.  The Journal of Wildlife Management 57: 771-778.

Elliott, K.H, J.E. Elliott, L.K. Wilson, I. Jones, and K. Stenerson 2011.  Density-dependence in the survival and reproduction of Bald Eagles: linkages to chum salmon.  The Journal of Wildlife Management 75: 1688-1699.

Farmer, C.J., L.J. Goodrich, E. Ruelas I., and J.P. Smith  2008.  Conservation Status of North America’s Birds of Prey.  In K.L. Bildstein, J.P. Smith, E. Ruelas I., and R.R. Veit (eds). State of North America’s Birds of Prey.  Nuttall Ornithological Club and American Ornithologists.  Union Series in Ornithology No. 3. Cambridge, Massachusetts, and Washington, D.C., 303-420.

Grubb, T.G., L.A. Forbis, M. McWhorter, and D.R. Sherman 1988.  Adaptive perch selection as a mechanism of adoption by a replacement Bald Eagle.  The Wilson Bulletin 100: 302-305.

Hancock Wildlife Foundation Forum.

Hornby Eagle Group Projects Society.  Our Nature Zone.

Hunt, W.G. 1998.  Raptor floaters at Moffat’s equilibrium.  Oikos 82: 191-197.

Institute for Wildlife Studies.  Channel Islands EagleCAM Forum.

Jenkins, J.M. and R.E. Jackman 1993.  Mate and nest site fidelity in a resident population of Bald Eagles.  The Condor 95: 1053-1056.

JudyB.  Watching Eaglets Grow.

Mahaffy, M.S. and L.D. Frenzel 1987.  Elicited territorial responses of northern Bald Eagles near active nests.  The Journal of Wildlife Management 51:551-554.

Markham, A.C. and B.D. Watts.  Documentation of infanticide and cannibalism in Bald Eagles.  Journal of Raptor Research 41: 41-44.

Mougeot, F. 2004.  Breeding density, cuckoldry risk and copulation behaviour during the fertile period in raptors: a comparative analysis.  Animal Behaviour 76: 1067-1076.

Mougeot, F., J. Gerrard, E. Dzus, B. Arroyo, P.N. Gerrard, C. Dzus, and G. Bortolotti 2013.  Population trends and reproduction of Bald Eagles at Besnard Lake, Saskatchewan, Canada 1968-2012.  Journal of Raptor Research 47: 96-107.

Partners in Flight.

Turrin, C. and B.D. Watts 2014.  Intraspecific intrusion at Bald Eagle nests.  Ardea 102: 71-87.

Turrin, C. and B.D. Watts 2015.  Nest guarding in Chesapeake Bay Bald Eagles.  Journal of Raptor Research 49: 18-28.

U.S. Fish and Wildlife Service.

Watts, B.D., G.D. Therres, and M.A. Byrd 2007.  Status, distribution, and the future of Bald Eagles in the Chesapeake Bay area.  Waterbirds 30: 25-38.

Watts, B.D., G.D. Therres, and M.A. Byrd 2008.  Recovery of the Chesapeake Bay Bald Eagle nesting population.  The Journal of Wildlife Management 72: 152-158.