Bald eagles lay their eggs in mid to late May in southern areas of Alaska, although one study in Southeastern Alaska indicated that they might lay their eggs as late as early June. The female lays between one and three off-white-colored eggs in a span of one to three days. The eggs range in size from 2.76 inches by 2.09 inches (70mm by 53 mm) to 3.31 inches by 2.36 inches (84 mm by 60mm).
The eagle pair begins incubating the eggs as soon as they are laid. The male and female share the incubation duties, and each mate hunts for its own food. Studies show that for 98% of the day, either the male or female sits on the eggs. The incubating bird stands up about once per hour and may change positions. A study at several nesting sites on Admiralty Island in Southeast Alaska found eggs were incubated for 95% of the daylight hours with females sitting on the eggs 53% of the time, and males tending the eggs 42% of the time. Brooding time dropped to 79% of the day for the first 10 days after the eggs hatched and by 41 to 50 days after hatching, the brooding time decreased to only 6% of each day. Brooding time increased when it was rainy and decreased when it was sunny. The incubation period takes 34 to 36 days. Since individual eggs may be laid a few days apart, they will not all hatch at the same time.
The hatching process is slow and arduous. It takes chicks twelve to forty-eight hours to fully emerge from the egg. The chick makes the first crack in the shell with its egg tooth, a small, hard bump on the top of the bill. After resting awhile, it then chisels around the large end of the egg. It eventually pushes off the end of the egg and wriggles out of the shell. The egg tooth dries up and falls off four to six weeks after hatching.
During hatching, a chick must undergo several physiological adaptations. Before it hatches, a chick absorbs oxygen through the mat of membranes under the shell. During the hatching process, it must cut the blood supply to these membranes and trap the blood within its body. At the same time, it must also inflate its lungs and begin breathing air once it has cracked the shell. The chick must also absorb the yolk sack into its body and seal off the umbilicus.
Newborn chicks are wet, exhausted, nearly blind, and extremely needy. Since a newly-hatched chick can’t regulate its body temperature, the parents must keep it warm. The chick is covered with pale gray down. The skin and scales of the legs are bright pink, the bill is a grey-black with a white tip, and the talons are flesh-colored. After the first week, the legs begin to turn yellow.
Eagles lay one to three eggs, but usually, only one or two chicks survive. Survivability is directly correlated to age. The first chick to hatch will be one to two days older than its siblings, so it will be larger and stronger and able to out-compete its nestlings if food is limited. If a brood has three chicks, the smallest chick usually dies within a week of hatching. Death is not normally caused by injuries from fighting with its siblings, but the chick simply starves to death because the older nestlings get all the food. The older chicks peck the young chick into submission to prevent it from eating enough to survive. By doing this, the older chicks ensure they have plenty of food.
___________________________________________________________________________
Robin Barefield is the author of three Alaska wilderness mystery novels, Big Game, Murder Over Kodiak, and The Fisherman’s Daughter. To download a free copy of one of her novels, watch her webinar about how she became an author and why she writes Alaska wilderness mysteries. Also, sign up below to subscribe to her free, monthly newsletter on true murder and mystery in Alaska.