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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.