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 4^th, 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.