For years, I have been collecting material for a Bald Eagle Glossary (dating back to my time as head mod at the Decorah eagles cam), but it took a back seat to other things. I have finally had time to work on it in earnest, and a few weeks ago I made a preliminary version of it live on my website. It is a work in progress, and probably will be for as long as I am able to maintain this website.

Glossary

Some entries are simple defintions. Others are supplemented with information that is relevant specifically to Bald Eagles. In many entries I have added details that explain things that cam observers have seen occur on nests, or that reveal aspects of Bald Eagle biology or behavior that only biologists know or think or wonder about.  One of my favorite things, etymology (linguistic origins of words), is included in some of the entries.  For example, under “photoperiod,” Greek phos = light.

I add new terms regularly and update entries as I learn more.  I welcome suggestions (and corrections from those who know more than I do), including requests for new entries.

On 12/29/23 observers of the Southwest Florida Eagle Cam in Fort Myers noticed a “dimple” or slight indentation on one of the two eggs, a change on the egg’s surface that suggested a hatch may be in progress. It was soon confirmed (by the pattern of stains on the 2 eggs) that this was Egg #2, laid on 11/27/23 at 13:44 (by my observation).  By the evening of 12/29, after about 32 days of incubation, the eggshell had a crack and some splintering along the side. By the next morning, 12/30/23, the shell was splintering on several sides and at the end. The outer shell membrane was visible between the bits of shell. The shell splintering continued throughout the day, and more of the membrane became visible. Observers could see movement by the chick inside the shell.

I (and others) have seen this kind of “messy” hatch (as I call it) on Bald Eagle nest cams several times over the years. Two shell membranes are fused to the shell and hold the shell fragments together. They are supple but fairly tough, so they can be difficult to break through. If the shell splinters in several places and the chick has a hard time breaking it apart, the membranes dry out, making them leathery and even tougher to split open. Most of the time the chick manages to wrangle a big enough space to emerge through the shell and membranes. But this kind of hatch is harder and more work than the more normal hatch where the shell breaks apart cleanly. In a few cases I have seen, it has proven too much for the chick, which becomes weaker from the effort and eventually fails to hatch.

But this case at SWFL was unlike the other “messy” hatches I have observed. In a normal hatch, the shell begins to crack open a day or two before the chick fully hatches. A shell breaking at 32 days of incubation, as at SWFL, might produce a hatchling at about 33 or 34 days. The earliest hatch in my records of Bald Eagle nest cams occurred after about 34 ½ days of incubation, and that was a third egg of a 3-egg clutch (third eggs usually hatch more quickly than first or second eggs), so the SWFL hatch would be a record early. (Stats on hatch timings are here.)

This suggested the possibility that the second egg’s shell was cracking prematurely, a suspicion reinforced by other indicators. One membrane around the inside of the shell is crucial to the embryo’s development (the chorioallantoic membrane, or CAM). It is full of blood capillaries, by which it effects the exchange of oxygen from the outside and carbon dioxide from the inside through microscopic pores in the shell. This membrane also stores metabolic wastes from the embryo and sheds them after the chick has hatched. (See more details on hatching here.) By the end of the day on 12/29 at SWFL, blood appeared on the shell and the membrane, which almost certainly came from the membrane’s capillaries. That raised some significant concerns about the health of the chick inside.

As most observers know, a day or two before the external pip in the shell, a swelling muscle in the chick’s neck contracts and pulls up the chick’s head toward the blunt end of the shell. There, it encounters an air cell, a space between the inner and outer shell membranes that contains a small amount of oxygen. The chick’s pipping tooth, which had formed along the top of its beak about a third of the way through the incubation period, pierces the inner shell membrane into the air cell. This is called the “internal pip.” It exposes the chick to air for the first time, which prompts the chick’s lungs and its nine air sacs to finalize their development and begin functioning. Over the next couple of days the lungs develop the ability to inhale oxygen and exhale carbon dioxide, gradually relieving the respiratory function of the chorioallantoic membrane, which begins to shut down.

The blood seen on the shell and membranes on the SWFL egg means that the chorioallantoic membrane’s capillaries were still performing the exchange of carbon dioxide and oxygen for the chick, that the internal pip probably had not yet occurred, and that the chick’s own internal respiratory system was not yet functioning. Whatever caused the shell to begin breaking, the damage to the membrane interrupted the transfer of respiratory function to the lungs, and the chick’s system probably began to experience oxygen deprivation and carbon dioxide build-up. This weakened the chick and increasingly rendered it unable to break through the dried-up shell membrane.

As difficult as this has been to watch, we can all be thankful that the first egg pipped on its normal timeline and had hatched by early this morning, 12/31/23.

See this page for more discussion of unhatched eggs.

There has been much consternation about what has been happening at this nest over the last few days. The new male, now named Beau (earlier V3), began a solid bond with the continuing female, Gabrielle (Gabby) last spring. She laid her first egg this season on 12/19/23. But Beau has not shown any interest in the egg, and on the evening of 12/22/23 he buried it.  Here is my current take on the situation.

I’ve seen eagle adults bury eggs several times over the 13 years I’ve been watching nest cams. The most recent example is at the Kisatchie E-3 nest in Louisiana, where the resident female (Andria) died on 12/9/23 and a new female appeared within hours. She covered the 2 eggs several times for a few days, but the male (Alex) has always uncovered them and is still incubating. Perhaps SWFL followers will remember a somewhat different situation in 2016, when M15 kept covering Harriet’s first egg in the days after it was laid, even though he wasn’t an intruder and had raised 2 fledglings with her the year before. He may have been shielding it from daytime heat while neither he nor Harriet was incubating. Harriet dug up that egg and laid another, although only 1 hatched. I’ve observed other instances as well, and I’ve also seen intruders destroy eggs or hatchlings (MD Blackwater in 2011, CA Redding in 2013, BC White Rock in 2013, WI E4K in 2016, OH Sandy Ridge in 2017, PA Hanover in 2018, FL NEFL in 2018, MN DNR in 2018).

So what is happening with V3/Beau? Let’s think through things, starting with whether or not Gabby’s egg was fertilized. Maybe, maybe not, there is no way to know. If it wasn’t, then the contents of the shell is just Gabby’s gamete, some yolk, and some albumen. If it was fertilized, then does Beau “knows” that the egg is “his” or, alternatively, does he suspect that it isn’t “his”? Well, birds don’t read biology books. They don’t know what “sperm” is. They don’t know what “fertilization” is. All they “know” is that they are stimulated to proceed through specific behaviors throughout the year, which lead to certain results – like the production of an egg. The question really doesn’t make sense for birds.

Why won’t he incubate? As I’ve explained before, because of the challenges by other males in the days leading up to Gabby’s egg, V3 was in fight mode. During the crucial pre-egg period, his reproductive hormones may have been keeping up with hers, but the challenges by other males threw his schedule off. Hormones from the Adrenal Glands (Corticosterone, Epinephrine, Norepinephrine) ramped up quickly. These hormones have a chemical make-up that enables them to have an almost immediate effect, unlike the reproductive hormones, which act more slowly. They stimulate various bodily systems (respiration, heart rate and blood flow, increased glucose and lipid production, suspension of digestion) to enable the eagle to respond to challenges quickly. These hormones also dampen secretion of the reproductive hormones. Meanwhile, Gabby’s system continued on its reproductive path, and she laid an egg on 12/19/23. Under typical circumstances, her reproductive hormones would remain active so that ovulation of a second egg could occur within a few hours after the first egg was laid. We don’t know yet whether that happened with Gabby, but if it did, that egg should have been laid yesterday evening. Or, her system may have perceived V3’s disinterest in her egg, and the resulting stress (exhaustion, hunger, etc.) may have induced her ovary to delay ovulation for a day. Or the ovary could have deposited the next mature ovum into her abdominal cavity instead into the oviduct, and it would thus not be fertilized and would be absorbed into the surrounding tissues.

In both females and males the brooding hormone, prolactin, begins a significant rise in secretion just before ovulation. For Gabby’s first egg, ovulation occurred three days before she laid it, on 12/16/23, and by the time she laid it, even while her reproduction hormones were still active in anticipation of the next ovulation, she also had ample prolactin in her system. Not so with Beau. The last recorded visit by another male was on 12/13/23. Adrenal hormones dampened the reproduction hormones during the stressful days, but as the adrenal hormones declined, his reproductive hormones and behaviors slowly began to emerge again as he perceived the situation to be calming down. But by that time his hormonal balance was hopelessly out of sync with Gabby’s.

Why did he bury the egg? I don’t think anyone can know the answer to that for certain. But 12/19/23 marked a huge change in Beau’s experience. Throughout that day he and Gabby were adding materials to the nest, chatting with each other, and flying and perching together, and he “practice” incubated in the nest cup for a while. At about 6 pm, everything turned upside down for him. Suddenly Gabby stopped perching and flying with him and instead spent hour after hour in the nest cup. If this is Beau’s first attempt at breeding, as many people believe, he may never have seen an egg before. He may have had no idea what it was or where it came from. And unless he had raised a clutch of eaglets in prior years, he would have had no clue that this thing might contain a tiny collection of cells that would steadily grow until one day that hard white shell would break apart and out of it would emerge a living, breathing, cheeping, hungry chick. All he knew was that Gabby’s behavior was bewildering, and that somehow it was connected with that large white thing that she kept spending all her time on top of, ignoring him except to yell at him. The times we saw him nosing around in the fluff around the egg, from the morning after it appeared until he finally and definitively buried it on 12/22/23, I think he was simply trying to restore things back the way they were. I don’t think he was trying to harm the egg, there is no reason to believe that he thought it might contain anything alive or threatening or edible. I think he was just trying to make it go away.

If Gabby continues to incubate, even if much less than before, prolactin is still flowing in her system. If she eventually decides to stop incubating altogether, her prolactin will recede and her reproduction hormones could stimulate another ovulation for a second clutch.

ADDENDUM: Gabby laid her second egg on 12/23/23 at 17:15. So she did delay ovulation for a day. Time will tell whether Beau’s hormones have caught up to hers — or hers have backed up to his — and he is able to incubate. And whether this egg is fertilized.

A few years ago, after watching Bald Eagle cams for a long time, I began to realize how central hormones are to the reproductive processes of birds. So I embarked on what turned out to be a lengthy journey of studying the topic. The journey took me back to observations, statistics, and videos I have collected over the years, and through hundreds of articles and books. I am still certain that I don’t understand everything. But I felt confident enough to put into words, charts, diagrams, and images at least some of what I was learning. The new pages I have created have helped me address many of my own questions about why things happen, and I have seen other folks who watch the eagles ask many of the same questions.

As I began to write, I found that the topic was much more immense and complex than simply the names of the hormones and what they do. So the writing morphed into a giant task of organizing into topics many facets of the biology and behavior of Bald Eagles that rest on the foundation of hormones. I have come up with four pages, each devoted to one of those topics:

The Avian Endocrine System (i.e., Hormones!)
Life History or Annual Cycle
Reproduction and Hormones
Photoperiodism and Life History

These pages are found under the Biology tab, along with References I’ve consulted.

The pages are rather long and detailed, which those nerdy Bald Eagle observers who have wondered about many of the same things I have may appreciate. But for those who don’t want so much information all at once, I have provided sidebars with Summaries of each page, as a guide to what the full text covers.

As always, I welcome comments and questions (through the Contact tab on this website, or on Facebook), and would be delighted to hear from anyone who knows more about hormones than I do with any corrections or clarifications that I need to make.

The Bald Eagle breeding season has begun — no eggs as of today, but lots of courtship and nest-tending is going on, especially in the southern tier of the Bald Eagle population.  Streaming cams on nests in the so-called “humid subtropics” — 6 in Florida (including 2 new ones), 2 in Louisiana, and 1 in Texas — are giving views of eagles active especially early in the morning and late afternoon through evening. Some of the eagles have stayed in or near their nests overnight, some have brought food to the nests, and some have been seen or heard (or both) copulating.

Now that the egg-laying season is almost upon us, I have created a new Nest Watch page for 2021-2022, where I will record dates of eggs, hatches, and fledges as they occur. Last year’s nest watch page has been moved to the Breeding tab.

To help those who monitor many nests to anticipate when eagles at the various nests will lay eggs, I have updated my Egg-Laying Calendars, monthly tables of dates at all the online nests from 2007 through summer 2021.

And I am trying to stay abreast of the cams that are streaming and those that aren’t, updates of links, and new cams (of which I know of 4) for the Links to Streaming Cams page.

Last year the first eggs at nests with cams were laid in the first week of November.  Um, that’s next week!

See you in the blinds!

Fifteen years of video cameras on Bald Eagle nests from 2006-2020 provide a wealth of information on eggs, chicks, fledges, clutches, and broods, numbers that beg for statistical analysis. I’ve spent several weeks (well, months, actually) crunching numbers for each of those years and have come up with several pages of tables of stats accompanied by summaries and discussion.

An introductory page sets the stage with an overview of the nests included in my study, 401 breeding efforts at 85 nest locations across North America over 15 years.

The first page of data in the series, Eggs, Nestlings, and Fledglings, presents a Table with all the numbers and a narrative analyzing them. Teasers:

• • 2-egg clutches were in the majority, making up almost 61% of all clutches. Only about half that number, 31%, had 3 eggs, and fewer than 7% had 1 egg.
  • 3-egg clutches were the most successful in producing at least 1 hatchling, while 1-egg clutches were least successful.

The next page, Success Rates of Clutches and Broods, further explores the clutches of eggs and broods of eaglets and how successful they were, with a new Table showing the numbers. Teasers:

• • More than 87% of the clutches overall produced at least one hatchling — a “successful” clutch.  All of the eggs hatched in more than 66% of them — what I am calling a “perfect” clutch.
  • Of broods of eaglets, nearly 77% were “successful” in producing at least one fledgling. An encouraging 47% of the broods resulted in all of their eaglets fledging — a “perfect” brood.

A new page digs down into Lost Eggs and Failed First Clutches of Eggs, with 2 Tables of data. Teasers:

• • Just under 21% of all the eggs were lost.
  • Over 55% of the lost eggs occurred in 1-egg clutches.
  • The cause was unknown in more than half of the lost eggs. Of the known causes, intruders at the nests made up the highest percentage (almost 13%).
  • A little more than 11% of the clutches of eggs produced no hatchlings. Almost half of these were failures of 1-egg clutches.

The following page, with 2 more Tables, similarly surveys Lost Nestlings and Failed Broods of Eaglets. Teasers:

• • Of all nestlings, almost 21% did not fledge.
  • A large plurality of losses of nestlings had no obvious cause.
  • Unlike with lost eggs, the largest percentage of known causes of lost nestlings was not nest intruders but bad weather.
  • The number of failed broods of eaglets was relatively low: Only about 9% of the broods of eaglets lost all of their nestlings before they could fledge.

These data do not show consistent trends from year to year, either in numbers or in percentages. Some years are more successful in hatches and fledges than others. Nothing in the data suggests that things are getting better or worse for Bald Eagle breeding.

UPDATE 12/3/2020:
I have created an entirely new page devoted to Second Clutches, with charts and data and research, and an overview of known Second Broods after the loss of eaglets.  I now know of 3 instances of the latter phenomenon among Bald Eagles, preceding the events at the Southwest Florida nest last year.
The new page is here.

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Here is the original post I published on 4/1/2020.

I’ll admit, I was as surprised as anyone on February 22, 2020 when the eagle pair at the Southwest Florida nest produced a second clutch of eggs after losing their eaglet 27 days after it hatched.  I know of the 8 instances of double clutches seen on streaming cams from 2006-2019, but all of those occurred after the loss of eggs, not eaglets.  Nothing I had read or heard of indicated that Bald Eagles would lay a second clutch of eggs after losing an eaglet.

Until today.

The original first egg at the Southwest Florida nest was laid on 11/12/19 and hatched on 12/19/19. The second egg in that first clutch, laid on 11/16/19, did not hatch.  After the eaglet’s death the parents continued to hang around the nest, bringing in new nesting materials, and they engaged in bonding behaviors and copulation.

On 2/22/20, 38 days after the loss of the first eaglet, the female parent, called Harriet, laid another egg, starting a second clutch, and 3 days later on 2/25/20 she laid a second egg.  The parents then incubated both eggs for the next 37 1/2 days, and on 3/31/20 the first egg of the new clutch hatched.  As of today the second egg has a large hole and the second eaglet seems to be making progress toward hatching.

It turns out that reclutching by Bald Eagles after loss of an eaglet is not unprecedented. I have been doing research on the success and failure of Bald Eagle clutches, and today I came across an article by A.L. Bryan, Jr., L.B. Hopkins, C.S. Eldridge, I.L. Brisbin, Jr., and C.H. Jagoe, published in 2005 in the journal Southeastern Naturalist. The article, titled Behavior and Food Habits at a Bald Eagle Nest in Inland South Carolina, reports observations at a nest in south central South Carolina in 1997, 1998, and 1999.

In the 1997-98 breeding season the nest produced 2 eaglets which fledged in February 1998. The following November the parents began their new breeding season.  I quote from the article:

"In November 1998, the eagle pair initiated another breeding attempt at Pen Branch and at least one nestling was presumed to have hatched (a feeding was observed on 9 December 1998). The pair abandoned this breeding attempt approximately 7 days later for unknown reasons (adult eagles were observed setting on the nest, either incubating eggs or brooding young, on 10 and 14 December). The pair initiated a second breeding attempt at Pen Branch in late February 1999 that produced 2 nestlings. These nestlings were discovered on the ground in dense vegetation by our personnel on 20 May and 7 June 1999, at approximately 55 and 73 days of age. These birds were taken to the South Carolina Center for Birds of Prey where they remained through August 1999, when they were released north of Charleston, SC."

The eaglet from the first clutch was only a few days old when the parents abandoned the nest, whether because the eaglet did not survive, or for some other reason. The authors of the article speculate that the female parent may have been a new mate and was perhaps not experienced in caring for a nestling. But their observations were not close or complete enough to be sure of the reason for the abandonment.

What is interesting from my perspective is that a re-clutch about 2 months after loss of an eaglet successfully produced two new eaglets. The reason why those eaglets did not fledge successfully on their own and ended up on the ground and in need of rescue is unknown.

But now I know that the events at the Southwest Florida nest this breeding season do not represent a first.