Chat Moderators > Decorah Eagle Mods Want You To Know -- Eagle Education

Growing Up...Developmental Milestones

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We are starting a new thread for discussion about the growing process of bald eaglets and the developmental milestones to watch for.  A quick reminder:  As has been stated in previous "Posting Guidelines," we ask that your posts and comments are limited to this topic in this particular thread.

I am entering FinnBMD's starting post on Finn's behalf.

In the first week of life, the hatchling becomes a nestling.  The growth and development process starts.  Initially the little ones are virtually helpless and their activities are minimal:  eating, sleeping and pooping.  For the first couple of days after hatch, they will be confined to the egg cup but by the end of that first week their growth will have started.

The late Canadian researcher, Gary Bortolotti, was a key figure in studying bald eagles and their offspring.  In one of his earlier articles, "Criteria for Determining Age and Sex of Nestling Bald Eagles," he wrote the following about the appearance of chicks in the first week after hatching:  "For the first few days after hatching the chicks can perhaps be aged by appearance.  On the day of hatching the area of the body where the allantois is attached is swollen, circular shaped and somewhat protruding.  Remnants of the allantois can be seen on the chick for up to 3 days after hatching, but after day 0 it is desiccated and thread-like.  On the day of hatching the chicks generally appear very weak and spend most of their time lying in the nest.  The skin is initially loose, wrinkled, and a bright pink, but it fills out, is noticeably tighter, and fades to a soft pink on day 1.  The legs too are very pink on day 0, but are a faded flesh color on day 1.  Beginning as early as days 2 and 3, but usually on day 4 or 5, the skin (upon close examination) on the ventral side [i.e., the front side of the body] is tinged with blue.  In the next 4 or 5 days the skin over most of the body is blue-gray."

In another article, Bortolotti wrote:  "On the day of hatching, eaglets had dark brown eyes, the gape and legs were pink, and the skin was bright pink.  The cere was a very pale gray and the culmen [the central midline ridge running from tip of upper bill back to base of the bill] was dark gray-black with a white tip.  The talons are largely flesh-colored. Between day 4-8 the skin turned from largely pink to largely blue except for a small area under the wings.  When the eaglets were 4-8 days old, the cere was pale yellow, the bill was dark gray-black and the legs were pink yellow.

Though this is a poultry site, my guess is that it is anatomically similar.  The day by day steps include the allantois and allantoic vessel

Finn, thanks for the great explanation of the first week of a young bald eagle hatchling. 
Going into the second week of life, the young nestlings start to become more curious though still completely dependent on their parents for warmth and protection against the elements.  They are not yet able to thermoregulate ( the ability to keep body temperature within certain boundaries regardless of ambient temperatures).   Commonly, the chick will rest at its parents breast or under it for warmth during this time and parents will never leave it unattended, though during more comfortable weather, the parent may not be physically on the nest constantly.
At about one and a half to two weeks of age most young weigh between one and a half to two pounds.   From nine to twelve days old, their cere (fleshy covering at the base of the upper mandible) is a pale olive color and legs are a pale yellow color with areas still pink-yellow.  When properly brooded, cold temperatures are not as significant a problem as heat is.  Often during warmer days the parent will spread its wings with its back to the sun to shade its young.  At about 10 days of age, a darker medium-gray thermal down (second down) begins to replace the natal down.  Thermal down has very good insulating qualities and by 15 days old the chicks are typically able to thermoregulate on their own.
Eagles have what is called an asymptotic pattern of growth in that they begin slowly, have a rapid phase of growth, followed by a slowing of growth to adult size.  Researcher Gary Bortolotti stated that male eaglets had an average growth rate of roughly 3.5oz per day while females average 4oz per day.  ( The bald eagle's growth rate is faster than any other bird in North America).
At two weeks old the eaglet is strong enough to hold its head up for feedings, and look around the nest area, and begging calls become louder and stronger.  Before two weeks of age the nestling will defecate in the nest but at about the two week mark, it will start to instinctively back up to the edge of the nest and release its excrement (which we affectionately refer to as PS) over the side. From early on in nests with more than one chick, sibling rivalry becomes apparent, partly due to competition for food and partly for parental attention with one bird, usually the oldest, out-competing or dominating the others.  The younger bird(s) learns to keep a low profile until the oldest has been fed but quickly learns to grab food or doing run arounds to the dominating eaglet to get its share.  This behavior occurs more within the first two weeks of life, declining somewhat as the siblings grow and learn.  They don't always eat every meal offered to them and one may eat while another (or 2) don't eat each time but they all  will normally get enough food during the course of the day.  On several occasions, we've seen dad feed the youngest while mom takes care of the other two!

Abbie, since male eagles and other male raptors are smaller, they reach their terminal size faster than females (at younger ages). Males make 1st flight at about 78 days (range 68 to 84) and females usually fledge later at about 82 days (range 75-88) - Gary Bortolotti.

He also stated that males mature faster, become more proficient hunters sooner than females and are more likely to disperse from natal nest area sooner than females.


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