The piercing, high-pitched call causes me look up at the sky, where I see two bald eagles flying in circles several hundred feet above my head. One eagle dives at the other. They touch talons and then separate and soar higher. A few minutes later, one eagle again dives at the other, and this time their talons lock, causing the birds to spiral downward in a motion much like a cart-wheel. I hold my breath as they plummet toward earth, but they finally pull away from each other and fly in opposite directions.
I assume I’ve just observed a courting display, and maybe I have, but biologists have a tough time differentiating between the aerial acrobatics of courting and those of aggression, and it is believed that the two behavioral displays may be closely related. Eagles also display other much-less aggressive forms of courtship, such as perching beside each other on a branch and stroking and pecking at each other’s bills. They also use their bills to stroke the heads, necks, and breasts of their mates.
Bald eagles sometimes mate for life, but if one member of the pair dies, the surviving member will often find a new mate within a year, and if a pair does not produce offspring after several seasons, they may change mates.
Copulation usually takes place near the nest site, and females lay between one and three eggs in mid-May on Kodiak. The male and female take turns incubating the eggs, and each mate hunts for its own food. The incubation period lasts 34 to 36 days, and it takes 12 to 48 hours for a chick to fully emerge from the egg.
During hatching, a chick must undergo several physiological adaptations. Before it hatches, the chick absorbs oxygen through the shell by way of the mat of membranes under the shell. During the hatching process, the chick must cut the blood supply to these membrane 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.
A newborn chick is helpless and dependent on its parents. It cannot regulate its body temperature, so the parents must keep it warm. Chicks grow rapidly as long as the parents can supply adequate food, and as the chicks develop and grow, the parents spend less time at the nest and more time hunting for food.
By the age of two weeks, most young eagles weigh one to two pounds (500 to 900 grams). Between 18 to 24 days, chicks gain four ounces (100 to 130 grams) per day, the fastest weight gain of any stage of their development. They begin feeding themselves by the sixth or seventh week and can stand and walk around the nest when they are eight weeks old. At sixty days, eaglets are well-feathered and weigh 90% of their adult weight.
Chicks remain in the nest for ten to twelve weeks, and we often see fledglings making their first flights in late August. Most nests where we live are located near the tops of tall cottonwood trees, and I wonder what it must be like for a young eagle to take that first step out of the nest. Their first flights are often very clumsy and quite humorous to watch as they learn how to use their huge wings to fly and master landing on a branch. Juveniles have longer wings and tails than adults, which makes it easier for them to learn how to fly, but it takes a while before they hone their skills, and they make several crash landings before they figure it out. They often continue to receive food from their parents while they learn to fly and hunt.