Tag Archives: data

ALL TEED UP FOR 2022-2023!

Egg time is fast approaching, and nests in Florida, South Carolina, Tennessee, Louisiana, Georgia, and many sites further north are busy with eagles visiting, rebuilding, and bonding.  Hurrricane Ian did significant damage, but reports and photos from the ground in Florida show bonded pairs checking out their old nest sites and, at least at the North Fort Myers nest, rebuilding the nest from scratch.

The first egg laid on cam last year was on November 12, but I have a record of an egg laid on November 2 (Northeast Florida in 2017).

I’ve moved the final 2021-2022 Nest Watch spreadsheet to its new location with other past seasons and started a new spreadsheet for the 2022-2023 Nest Watch.  Egg-laying Calendars from 2008-2022 are updated to provide some guidance on when to expect eggs in various regions across North America.

Here is a page with Links to all the current Streaming Cams.

Happy eagling!

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

Click on the Pop-Out button to open in a new tab.

Loader Loading...
EAD Logo Taking too long?

Reload Reload document
| Open Open in new tab

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

Click on the Pop-Out button to open in a new tab.

Loader Loading...
EAD Logo Taking too long?

Reload Reload document
| Open Open in new tab

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

Click on the Pop-Out button to open in a new tab.

Loader Loading...
EAD Logo Taking too long?

Reload Reload document
| Open Open in new tab

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

Click on the Pop-Out button to open in a new tab.

Loader Loading...
EAD Logo Taking too long?

Reload Reload document
| Open Open in new tab

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

Click on the Pop-Out button to open in a new tab.

Loader Loading...
EAD Logo Taking too long?

Reload Reload document
| Open Open in new tab

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

Click on the Pop-Out button to open in a new tab.

Loader Loading...
EAD Logo Taking too long?

Reload Reload document
| Open Open in new tab
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

Click on the Pop-Out button to open in a new tab.

Loader Loading...
EAD Logo Taking too long?

Reload Reload document
| Open Open in new tab

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.

 

BALD EAGLE NEST WATCH 2023-2024

NESTEGGSHATCHESFLEDGESNOTESCOUNTUP
(italics = anticipated)
FL cen
clutch 1		11/2/23
11/5/23		One egg broken 12/8.
Other egg broken 12/15
FL cap11/11/23
11/14/23		12/17/23
12/20/23
3/13/24		H1 d. 2/16/24, cause unk.
F2: 49 days 3 hours 31 minutes
LA kf311/18-19/23
11/22/23		inadequate incub.F became ill 12/7, fell from nest 12/8 pm, found deceased, necropsy revealed rodenticide poisoning.
Several interested Fs have visited. F3 buried eggs 12/27
FL swf
new F		11/24/23
11/27/23		12/31/23
hatch fail		3/16/24 (accid) F1: 46 days 1 hour 52 minutes
FL mdc12/6/23
12/10/23		1/14/243/30/241 egg unhatched, unk which F1: 32 days 25 minutes
GA ber
clutch 1		12/8/23
12/11/23		both eggs broken1 egg broken overnight 1/20-1/21, other egg collapsed 1/21 <21:15
LA kf112/9/23inadequate incub.F became ill 12/27. Grounded & rescued 12/31, d. 1/1/24. Floaters in the area
TX jbs12/18/23
12/21/23		1/26/24
<1/28/24		4/14/24H2 crawled outside nest cup 1/31, turned on back, unable to crawl back to cup. F1: ~ 17 days 3 hours 37 minutes
FL nef
new M		12/19/23

12/22/23		buried, incub. fail
unhatched		E1 buried deep by M on 12/22.
Floaters continue in nest area.
New M incubated E2 a few times.
FL ece
new nest		12/25/23
12/28/23		<1/31/24
<2/2/24		H2 blown off nest 4/3, on lower limb, grounded, rescued 4/5. Euthanized 4/15. F1: 7 days 4 hours 16 minutes
AZ gfd
new M		<1/13/24<2/20/24
<2/23/24?
<2/27/24?		Incubation observed 1/13.
Feeding observed 2/20.
2 eaglets seen 3/2.
3 eaglets seen 4/8.		dates unk
FL cen
clutch 2		1/13/24
1/16/24		2/20/24
2/22/24		 H1: ~ 71 days 4 hours 20 minutes
H2: ~69 days 7 hours 23 minutes
TN jns
1/18/24
1/22/24		2/25/24
2/27/24		 H1: 66 days 1 hour 13 minutes
H2: 63 days 23 hours 20 minutes
NJ duk1/22/24
1/25/24		2/29/24
3/1/24		 H1: ~ 61 days 15 hours 43 minutes
H2: 60 days 16 hours 20 minutes
CA bbl1/25/24
1/28/24
1/31/24		unhatched
unhatched
unhatched
TN blf1/28/24
1/31/24		3/6/24
3/8/24		Intruders disrupted food provision; H2 d. 3/23. H1: 55 days 14 hours 7 minutes
CA cWE1/30/24
2/2/24?
2/5-6/24?		3/9/24
3/9/24
3/12/24		New nest 2022-23.
Cam down 2/1-2/13.
Three eggs in view 2/14		 H1: ~ 53 days 5 hours 35 minutes
H2: ~ 52 days 11 hours 51 minutes
H3: 50 days 5 hours 34 minutes
CA zSC
clutch 1		2/2/24E broke 3/16/24, no embryo seen.Cam down 2/1-2/16
MD mas2/4/24
<2/9/24		(3/12/24)Cam offline overnights. E1 in view ~07:04 on 2/5.
Cam offline for season.
PA frm2/8/24
2/11/24
2/14/24		3/19/24
3/20/24
3/22/24		 H1: ~ 43 days
H2: ~ 41 days 17 hours
H3: 40 days 5 hours 59 minutes
MD tob2/8/24
2/11/24
2/14/24

3/23/24		1 E broke 3/13, no embryo seen.
1 E hatch fail 3/18.
M missing 4/9-

H3: 39 days 4 hours 56 minutes
OH cin2/13/24
2/16/24		3/21/24
3/23/24		 H1: 41 days 9 hours 3 minutes
H2: 39 days 7 hours 41 minutes
KS der
new nest		2/13/24
2/16/24		3/21/24
3/23/24		 H1: 41 days 6 hours 29 minutes
H2: 39 days 5 hours 7 minutes
VA dul
new M		2/14/24
2/17/24		M not incubating.
Abandoned 2/23; crows attempted predation.
2/26 new pair appeared, buried the eggs.
CA red2/15/24

2/18/24
2/21/24		broke 2/29
3/26/24
3/29/24
H2: ~ 35 days 18 hours 33 minutes
H3: ~ 33 days 4 hours 43 minutes
IA dnn2/15/24
2/18/24		<3/24/24
<3/25/24		 H1: ~ 38 days 11 hours
H2: ~ 37 days 8 hours 29 minutes
OK brt<2/17/24
<2/20/24
2/23/24		3/25/24
3/27/24
3/30/24		cam online 2/20.
Offline 3/14-3/22.		 H1: ~ 37 days 1 hour 43 minutes
H2 time unk
H3: ~ 31 days 16 hours 32 minutes
GA ber
clutch 2		2/17/24
2/20/24		3/25/24
unhatched		H1 d. <4/11, cause unk, lethargic, respir. distress
OH lmc2/19/24
2/22/24
2/25/24		3/28/24
3/29/24
4/2/24		 H1: ~ 34 days 11 hours 43 minutes
H2: ~ 32 days 19 hours
H3: ~29 days 10 hours 43 minutes
IA urb2/19/24
2/22/24
2/26/24		3/30/24
3/31/24
4/3/24		 H1: ~ 32 days 4 hours
H2: ~ 31 days 2 hours 36 minutes
H3: 27 days 22 hours 20 minutes
CA fol2/19/24
2/22/24
2/26/24		3/27/24?
3/28/24?
<4/4/24		H3 fell below nest 4/27, rescued, in rehabH1 time unk
H2 time unk
H3: ~ 27 days 11 hours 43 minutes
NY ctr
new M		2/20/24
2/24/24
2/27/24		<3/30/24
<4/1/24
<4/6/4		Feeding H1 observed 3/31.
H2 seen 4/1 a.m.
H3 seen 4/6 06:47.		H1 time unk
H2 time unk
H3 time unk
PA pit
new M		2/20/24Egg collapsed 3/18 ~21:39
IA dav2/22/24
2/25/24
2/29/24		<3/31/24
<4/5/24
unhatched		H1 seen 3/31 ~03:50
H2 seen 4/5 07:45		H1 time unk
H2 time unk
MI arb2/22/24
2/25/24		3/30/24? E1: 68 days 17 hours 57 minutes
E2: ~ 65 days 18 hours 57 minutes
WI chp2/22/24
2/25/24		unhatched
unhatched
CA cTH2/22/24unhatched
CA zFP
new nest		2/23/24
2/26/24
2/29/24		4/2/24
4/4/24		1 E broke <3/1, unk which H1: ~ 29 days 35 minutes
H2: 27 days 2 hours 56 minutes
PA han
new F & M?		2/24/24

2/27/24		unhatched
4/8/24		intruder, incub. disrupted.
M killed H2 4/8 11:04
IN ndl
new F		2/24/24
2/27/24		4/2/24
4/3/24		 H1: ~ 29 days 2 hours 43 minutes
H2: ~ 27 days 21 hours 13 minutes
WI trm
new cam		2/25/24
2/29/24		4/3/24
4/7/24		M missing 3/28-4/6. H1: ~28 days 11 hours 42 minutes
H2: ~23 days 22 hours 13 minutes
WV shp
new M		2/27/24

3/1/24
3/4/24		broke 3/19
4/7/24
4/11/24		E1 broke 3/19 <07:23
M killed H2 4/8 17:43, and H3 4/12 08:45.
ME pis2/28/24
3/2/24		<4/9/24Feeding seen 4/9 14:49.
No feeding since 4/16, no brooding		H1: date/time unk
E2: ~ 59 days 18 hours 43 minutes
OH avn2/29/24
3/3/24
3/6/24		4/9/24
4/10/24
4/11/24		H1 seen 4/9 06:42
H2 seen 4/10 07:30		 H1: ~ 22 days 8 hours 43 minutes
H2: ~ 21 days 7 hours 43 minutes
H3: 19 days 23 hours 39 minutes
CO fsv2/29/24
3/3/24
3/7/24		4/9/24
4/9/24
4/13/24		H3 fell on back 4/14, unable to turn over, likely asphyxia H1: 22 days 3 hours 37 minutes
H2: ~ 21 days 19 hours 48 minutes
PA wmf3/1/24
3/5/24		4/7/24
unhatched		 H1: 24 days 2 hours 38 minutes
CA mBC3/2/24?
<3/5/24		4/8/24
<4/10/24		Cam online 3/4, offline most of 3/5
H2 seen 4/10 07:58		 H1: ~ 22 days 14 hours 33 minutes
H2 date/time unk
BC wht3/9/24
3/12/24		4/15/24
4/17/24		 H1: 16 days 3 hours 44 minutes
H2: 14 days 5 hours 21 minutes
BC bbc3/21/24
3/24/24		4/26/24
4/28/24		 H1: 4 days 12 hours 50 minutes
H2: 2 days 19 hours 18 minutes
BC sur3/27/24
3/29/24		(5/3/24)E2 laid ~20h too early, shell soft E1: 34 days 19 hours 50 minutes
CA zSC
clutch 2		4/11/24
4/15/24
4/18/24
4/21/24
4/26/24		broke
broke
broke
broke
broke
WHO'S NEXT?LAST YEAR FIRST EGG
AK gla4/18/23
4/22/23
NEW,
RESTART,
NEW NEST		(in approximate chronological order)
NJ 3br
MN alb
new nest & cam
NESTS INACTIVE OR NOT ONLINE 2023-24
FL aubunk
unk		Not streaming.
FL osc11/12/21Cam offline since Hurricane Ian.
SC hil11/19/21Great Horned Owls took over nest.
LA ben11/26/22
11/29/22		No breeding eagles in nest.
TX web
no live stream, videos on Facebook		12/6/22
12/9/22		No breeding eagles in nest.
TN dal1/23/23
1/26/23
1/29/23		Not streaming.
DC arb2/17/22Using alternate nest off cam.
OH esl2/24/22?Using alternate nest off cam.
IA dec2/25/23
2/28/23		Building new nest off cam.
MN dnr2/15/23
2/18/23		New nest not on cam 2023-2024
MT mil2/28/22Not breeding in nest on cam.
BC har

BALD EAGLE NEST CAM BREEDING SEASON 2019-2020

I use short codes to refer to each nest. Click here for a key.
Click here for links to the nest cams.
Click here for calendars of egg-laying dates in past years.
A question mark ? indicates an approximate date or best guess.
Strikout indicates unhatched eggs.
Italics indicate nestlings that died before fledge.
< = by this date.

More detailed information can be found in
JudyB’s annual report on the nests.

NESTEGGSHATCHESFLEDGESNOTES
FL swf
clutch 1		11/12/19
11/16/19		12/19/19
unhatched		d. 1/15/20
---		eaglet ingested anticoagulant poison, broken blood feather led to fatal blood loss
TX web
new M?		12/12/19
12/15/19		1/17/20
1/19/20		4/6/20
fell 4/7/20,
rehab,
released 4/23/20		cam not streamed live, videos posted on Facebook
FL ece12/28/19
12/31/19		2/3/20
2/5/20		5/1/20
5/7/20
OK seq
new M?		1/4/20
1/7/20		1 broke 2/8/20
1 abandoned <2/15/20		---
---		intruder adult attacked 2/8/20, Mom possibly injured; unk which egg broke, which abandoned
CA bbl1/8/20
1/11/20		unhatched
unhatched		---
---		ravens ate eggs 3/15/20
GA ber1/11/20
1/14/20		1 broke 2/22/20
1 broke 3/11/20		---
---		unk which egg broke first
FL nef
new F & M		1/13/20
1/16/20		2/19/20
2/21/20		5/7/20
5/17/20
NJ duk
new F		1/20/20
1/24/20		2/26/20
3/1/20		both seen 6/25/20cam down
4/30/20 thru
end of season
TN dal1/22/20
1/25/20
1/28/20		2/29/20
3/2/20
3/4/20		fell 3/4/20
1 seen <6/8/20
1 seen <6/15/20		tree fell 4/29/20, cam down
TN blf1/27/20
1/30/20
2/3/20		3/5/20
3/7/20
3/11/20		5/29/20
5/29/20
6/5/20
MD tob2/3/20
2/6/20

2/9/20		3/14/20
hatch fail, 3/14/20
3/16/20		6/4/20?
---

6/5/20?
TN jns2/4/20
2/7/20
2/10/20		3/15/20
3/17/20
unhatched		6/3/20
6/5/20
---		male (Noshi) disappeared 4/24/20; intruder M appeared on cam 4/26/20
MN dnr
new F & M		2/6/20
2/9/20
2/12/20		3/15/20
3/17/20
3/19/20		6/16/20
6/20/20
d. <4/13/20
E3 failed to thrive, possibly starved
AZ gfd
new nest
new M
clutch 1		2/9/20?------new M didn't incubate; nest
abandoned 2/18/20
MI arb<~2/16/20
?		3/20/20?
?		6/9/20
?
CA red
no cam		2/11/20?
?		3/22/20?
<4/12/20		6/17/20?
?
PA frm
new cam		2/13/20
2/16/20

2/19/20		3/25/20
hatch fail, 3/25/20
3/26/20		6/14/20
---

<6/16/20
PA han2/13/20
2/17/20		broke 3/29/20
broke 2/17/20		---
---
PA pit2/13/20
2/16/20		3/21/20
3/23/20		6/11/20
6/6/20
IL umr2/14/20
2/17/20		3/23/20
3/24/20		6/10/20
6/16/20		tree fell in derecho 8/10/20
CA robat least 2
<2/27/20		2 hatched
<3/25/20		<6/10/20
<6/10/20
CO fsv2/14/20
2/17/20

2/21/20		1 hatch 3/29/20
1 hatch <3/31/20
1 unhatched		1 d. ~4/16/20
<6/12/20		cams off 3/16/20 due to coronavirus; unk which egg unhatched; unk which chick d., probably weather related
WV shp2/17/20
2/20/20		3/25/20
3/27/20		fell 3/27/20
6/13/20
CA cWE
new M
clutch 1		2/20/20
2/23/20		ravens predated
both 2/24/20		---
---		new M didn't incubate consistently
IA dnn2/21/19
2/24/20		3/30/20
3/31/20		d. 4/10/20
6/9/20		DN11's death cause unk
CA zSC
new nest
clutch 1		2/21/20broke at lay---
FL swf
clutch 2		2/22/20
2/25/20		3/31/20
4/2/20		6/15/20 accid
7/1/20		102d after clutch 1 first egg, 38d after clutch 1 eaglet d.
NY ctr2/22/20
?
?		3/31/20
<4/2/20
<4/5/20		6/18/20?
6/20/20?
<6/26/20
IA urb
new nest		2/25/20
2/28/20
3/2/20		4/3/20
4/6/20
4/7/20		6/22/20
6/25/20
6/29/20
IA dec2/26/20
2/29/20
3/4/20		4/5/20
4/5/20
4/8/20		6/18/20
6/21/20 accid
6/21/20
IN ndl2/26/20
2/29/20
3/4/20		4/4/20
4/6/20
4/9/20		6/23/20?
6/26/20
d. 5/14/20
BC sur2/27/20
3/1/20		4/5/20
4/6/20		6/28/20
6/28/20
CA cTH2/27/20
3/1/20		4/5/20
broke 3/20/20,
infertile		6/26/20
---
MT mil3 by 3/6/20<4/5/20
<4/7/20
<4/14/20		<6/18/20?
<6/20/20?
<6/28/20?
CA zSC
clutch 2		2/29/20
3/4/20
3/7/20		broke in 3 hrs
broke <3/5/20
broke <3/11/20		---
---
---
OH avn3/1/20

3/4/20
3/7/20		4/10/20

4/11/20
4/12/20		6/18/20 accid (BDOW)
1 d. 4/26/20
6/29/20		unk which chick d. or cause
CO std3/1/20
3/4/20
3/7/20		unhatched
4/12/20
unhatched		---
d. 4/14/20
---
intruder F attacked 4/6/20; eaglet d. of hypothermia or starved; magpie took eaglet's body 4/15/20; M stopped incubating
BC dl23/1/20
3/4/20
3/7/20		1 unhatched
4/11/20
4/12/20		---
7/1/20 accid
7/4/20 accid		unk which egg unhatched
OH ash
new cam		3/1/20
3/4/20		4/7/20
4/10/20		6/24/20
6/24/20
IA dav3/6/20
3/9/20
3/12/20		unhatched
4/14/20
4/17/20		---
<7/4/20
<7/4/20		egg #1 gone 3/7/20?, unk cause
TN har3/9/20?
3/12/20?		<4/16/20
4/18/20		<7/6/20
<7/6/20
BC wht3/10/20
3/13/20		unhatched
4/18/20		---
7/2/20
AZ gfd
clutch 2		~3/10/20------new M didn't incubate; abandoned
<3/25/20
CA cWE
clutch 2		3/18/20

3/21/20		raven predated,
3/20/20
raven predated, 3/29/20		---

---
BC hrn3/28/20
3/31/20 ?		5/5/20
<5/9/20		7/28/20
8/2/20
AK ken4/22/20
4/25/20		5/31/20
6/2/20		8/19/20
d. from fall, 6/22/20		M (Redoubt) disappeared 6/18/20; possible intruder

MEASURING ADULT, SUBADULT, AND JUVENILE BALD EAGLES

©elfruler 2018

See MEASURING AN EAGLE for details on procedures and challenges of acquiring measurements and descriptions and figures of the features measured.   General References are given at this link, while References specific to each table below are given at the end of each table.

The charts below give measurements of adult and subadult Bald Eagles as reported in peer-reviewed publications.  I have omitted measurements that are questionable or not standard.  (If a reader knows of reports that I do not include here, please contact me with details.)

These numbers provide some context for consideration of several factors relating to size in Bald Eagles:

Age

The age of a Bald Eagle during its first five years affects several measurements.

  • Beak and talons increase in size.
    • A Bald Eagle’s beak and talons are not fully grown at fledge but increase slowly in size over approximately its first 3 years. This is probably caused by a gradual buildup of the keratin layer over the underlying bones (which do appear to be fully grown at fledge) (Bortolotti 1984d).
  • Feathers decrease in length.
    • A juvenile, a fledgling eagle in its 1st year, has longer flight feathers (wing and tail) than it will ever have again.
    • With each successive molt of a subadult from its 2nd year through its 5th, the new flight feathers are a few millimeters shorter.
    • After reaching maturity, feather lengths of adults remain steady. But feathers wear down over time:  An individual Bald Eagle primary or secondary feather molts only every 3-4 years so it becomes progressively shorter over that period.  Also, a new flight feather can take 40-50 days to grow to its full length, so a measurement before it has finished growing will be misleading.
  • Weight.
    • Primarily because of the decreasing feather lengths, an eagle’s weight decreases slightly over its 1st 5 years.
  • Researchers cannot always be certain of the age of a particular bird, and some offer vague or imprecise descriptions of age, such as “second winter,” “immature,” or “subadult.”
  • Only measurements of the same eagle from one year to the next would yield meaningful comparisons, and this is possible only with a captive bird or with nestlings that are visited more than once before they fledge. Few such measurements exist.

Sexual dimorphism

  • Females are larger than males in general, although exceptions can exist. The numbers tabulated here indicate that the difference can be from 13-23%, although some publications and internet sites claim as much as 25-30%.
  • Adult females are larger than subadult females.
  • Adult males are not significantly larger than younger males. (Bortolotti 1984c)

Geographical location and “Bergmann’s rule”

  • It has been stated often that the size of Bald Eagles increases from south to north, and the so-called “Bergmann’s rule” is cited as an explanation for this phenomenon. The numbers in my tables do not necessarily confirm this “rule” for Bald Eagles, as discussed below.
  • “Bergmann’s rule” has roots in an 1847 article by Carl Bergmann, entitled “On the relationship of the warmth economy of animals to their size” (trans. Salewski and Watt 2016).
    • Bergmann described a “law” pertaining to warm-blooded animals (birds and mammals):
      • Since larger animals have a smaller ratio of body surface area to body volume, they expend less effort than smaller animals to maintain a constant internal body temperature. (The surface area is important for the dissipation of heat from the body to compensate for excessive environmental heat, while the volume is pertinent to heat production to warm the body in excessive cold.)
    • From this “law,” Bergmann hypothesized that larger animals need a cooler climate than smaller animals. Since in general environmental temperature is lower at higher latitudes (further north), it follows that larger animals would favor northern environments and smaller animals would favor southern environments, a concept that biologists refer to as a latitudinal size cline, a gradation of size from larger to smaller, in this case from north to south.
    • Bergmann tested the size cline hypothesis by comparing the relative sizes of species within a genus, only once mentioning the sizes of individuals within a single species (the White-tailed Eagle, see below). Using wingspan (not body volume, or weight) to compare size, he examined 310 species across 86 genera and concluded that the hypothesis of a latitudinal size cline is true in most (but not all) cases.
    • To address the exceptions, Bergmann noted that other factors besides latitude might be in play:
      • Altitude (mountainous habitats generally are cooler than lower elevations).
      • The reliability of wingspan as an indicator of size (e.g. the Merlin has a smaller wingspan than the European Hobby but can be of comparable weight and remains in northern climates through the winter).
      • Migratory habits (which may affect wingspan).
      • Quality of plumage.
    • Bergmann included Bald Eagles (Haliaeetus leucocephalus) in a sea-eagle taxon with White-tailed Eagles (Haliaeetus albicilla), Short-toed Eagles, and Ospreys, although he acknowledged that in his day there was disagreement about whether they all belong in the same genus (taxonomists today agree that they do not).
      • He noted that among White-tailed Eagles, which conform to the hypothesis in general, some smaller individuals may be found in the north and some larger ones in the south.
    • Bergmann himself never articulated a “rule” about a relationship between the size of an animal and its geographical location. Later researchers have formulated the “rule” in different ways, and there remain disagreements about its underlying assumptions, its application, and even its validity.
    • A latitudinal size cline does not apply to all species and genera of birds. Meiri and Dayan 2003 surveyed studies of 94 species of birds that provide reliable data on size and locale and concluded that “over 72% of birds…follow Bergmann’s rule.”
      • Among raptors found in North America, that includes Turkey Vultures, Peregrine Falcons, Sharp-Shinned Hawks, and Ospreys.
      • But several North American raptors do not follow “Bergmann’s rule”: Cooper’s Hawks (Whaley and White 1994) , Northern Goshawks (Whaley and White 1994), Red-tailed Hawks (Fitzpatrick and Dunk 1999), and Merlins (Temple 1972).
      • Meiri and Dayan did not include Bald Eagles in their survey because not enough studies were available that provide “data that were statistically tested for geographic variation.”
    • My tables here do not provide such data for Bald Eagles either. The sampling in the published literature is too small, variable, and arbitrary to either confirm or refute “Bergmann’s rule” in the case of Bald Eagles.
    • In fact, the numbers I have tabulated suggest that, as with White-tailed Eagles, which are close genetic relatives of Bald Eagles, some smaller individual Bald Eagles may be found in the north and some larger ones in the south. The ranges of weight and wingspan measurements in my tables illustrate some exceptions to “Bergmann’s rule” (ranges are given in parentheses and italics below the averages):
      • The highest weight among females was recorded in Illinois (6577g) and the lowest in Saskatechwan (4540g) – higher in the south, lower in the north.
      • The highest weight among males was found in Alaska (5625g) and the lowest also in Alaska (3633g) – both high and low in the north.
      • The longest wingspan among females was recorded in Alaska (2333.5mm) and the shortest in Illinois (2035mm) – longer in the north, shorter in the south.
      • But the longest wingspan among males was found in Alaska (2171.7mm) and the shortest in Saskatchewan (2027mm) – both long and short in the north.
    • In conclusion, until more systematic studies of Bald Eagles are done with large samplings of measurements across a full range of geographic locations, we cannot be certain which of the following is true:
      • Bald Eagles are more similar to Turkey Vultures, Ospreys, Peregrine Falcons, and Sharp-Shinned Hawks in always or almost always conforming to “Bergmann’s rule,” or,
      • Bald Eagles are more similar to their sister White-tailed Eagles in following the “rule” generally, but having many individual exceptions. Our limited data suggests that this is a more accurate statement.

ADULT BALD EAGLE MEASUREMENTS TABLE

SUBADULT AND JUVENILE BALD EAGLE MEASUREMENTS TABLE

REFERENCES

MEASURING AN EAGLE: REFERENCES

Baird, S.F., T.M. Brewer, and R. Ridgway 1874.  A History of North American Birds: Volume III, Land Birds.  Little, Brown, and Co.: Boston.

Baldwin, S. P., H. C. Oberholser, and L.G. Worley 1931.  Measurements of Birds (1931).  Scientific Publications of the Cleveland Museum of Natural History, Cleveland, OH.

Bent, A.C. 1937.  Life histories of North American birds of prey: order falconiformes.  Smithsonian Institution, Washington, DC.

Bortolotti, G.R. 1984a.  Criteria for determining age and sex of nestling Bald Eagles.  Journal of Field Ornithology 55: 467-481.

Bortolotti, G.R. 1984c.  Sexual size dimorphism and age-related size variation in Bald Eagles.  The Journal of Wildlife Management 48: 72-81.

Bortolotti, G.R. 1984d.  Physical development of nestling Bald Eagles with emphasis on the timing of growth events.  The Wilson Bulletin 96: 524-542.

Broley, C.L. 1947.  Migration and nesting of Florida Bald Eagles.  The Wilson Bulletin. 59: 3-20.

Chura, N.J., and P.A. Stewart 1967.  Care, food consumption, and behavior of Bald Eagles used in DDT tests.  The Wilson Bulletin. 79: 441-448.

Fitzpatrick, B.M. and J.R. Dunk 1999.  Ecogeographic variation in morphology of Red-Tailed hawks in western North America.  Journal of Raptor Research 33: 305-312.

Friedmann, H. 1950.  The birds of North and Middle America: A descriptive catalog.  Part XI.  Smithsonian Institution, Washington, D.C.

Gerrard, J.M. and G.R. Bortolotti 1988.  The Bald Eagles: Haunts and Habits of a Wilderness Monarch.  Smithsonian Institution Press, Washington and London.

Gerrard, J.M., A.R. Harmata, and P.N. Gerrard 1992.  Home range and activity of a pair of Bald Eagles breeding in northern Saskatchewan.  Journal of Raptor Research 26: 229-234.

Imler, R.H. and E.R. Kalmbach 1955.  The Bald Eagle and its economic status.  U.S. Fish and Wildlife Service Circular 30.

Maestrelli, J.R. and S.N. Wiemeyer 1975.  Breeding Bald Eagles in captivity.  The Wilson Bulletin 87: 45-53.

Meiri, S. and T. Dayan 2003.  On the validity of Bergmann’s rule.  Journal of Biogeography 30: 331-351.

Palmer, R.S., ed. 1988.  Handbook of North American Birds vol. 4: Family Cathartidae, New World condors and vultures; Family Accipitridae (first part), Osprey, kites, bald eagle and allies, accipiters, harrier, buteo allies.  Yale University Press, New Haven and London: 187-237.

Salewski, V. and C. Watt 2016.  Bergmann’s rule: a biophysiological rule examined in birds.  Oikos 126: 161-172.

Southern, W.E. 1964.  Additional observations on winter Bald Eagle populations: including remarks on biotelemetry techniques and immature plumages.  The Wilson Bulletin 64: 121-137.

Stalmaster, M.  The Bald Eagle.  Universe Books, New York.

Temple, S.A. 1972.  Systematics and evolution of the North American merlins.  Auk 89: 325-338.

U.S. Fish and Wildlife Service Forensic Laboratory.  The Feather Atlas: Flight feathers of North American birds.

Whaley, W.H. and C.M. White 1994.  Trends in geographic variation of Cooper’s hawk and Northern goshawk in Northern America: a multivariate analysis.  Proceedings of the Western Foundation of Vertebrate Zoology 5: 161-209.

Wright, B.S. 1953.  The relation of Bald Eagles to breeding ducks in New Brunswick.  The Journal of Wildlife Management 17: 55-62.