Pennsylvania Game News - February 2012 - 53

pair interested in nesting or already nesting. After all, in Pennsylvania bald eagles return to their nesting grounds as early as December through February, when they engage in nest-building or repair, courtship and breeding. Most eggs are laid between mid-February and mid-March. Doug Gross, Endangered and Nongame Bird Section Supervisor for the Commission says that “bald eagles are still increasing with many miles of rivers still without a pair established. This is especially true in the southwest region. Pairs also are changing from one nest location to another, the second nest often more difficult to see, so we are probably missing some nests in yearly counts that are still active. Eagles often nest in locations difficult to see, especially after leaf-outs, including islands, hillsides and swamps.” Both Gross and Barber depend on the public to report nests, and Barber says that “Some of the latest [nests] reported were found by birders walking trails in remote or rugged locations.” These nests are usually high in large live trees, such as sycamores or white pines, close to a dead tree where they perch, and within a mile or so of water. Both sexes build their nest and take anywhere from four days to three months, interweaving sticks collected from the ground or broken off of nearby trees, lining it with finer woody materials and their own downy feathers. These nests are huge, among the largest of all birds, and are often reused year after year, by a species suspected to be monogamous and mated for life unless one mate dies. Courtship can only be described as ecstatic, featuring acrobatic flight displays, most notably the cartwheel display in which a courting pair flies high in the sky, locks talons, and tumbles down to earth, breaking off at the last moment to avoid hitting the ground. Other courtship displays include the chase display, when a pair pursue each other, occasionally lock talons, rolling and diving, and the so-called
FEBRUARY 2012

roller-coaster flight, when one eagle flies high, folds its wings, and dives directly to earth, swooping back up at the last moment to avoid hitting the ground. After all that excitement, followed by breeding, the female lays one to three, dull white eggs in the nest and begins incubating after she lays her first egg so the young hatch over a period of several days. The male helps with incubation, although his brood patch is not as well developed as the female’s. Both parents step gingerly around the eggs, which didn’t help when DDT thinned their eggshells, causing them to crack open prematurely. After 35 days, the first youngster emerges from its egg, followed by its siblings on subsequent days. Both parents hunt and feed the nestlings, but the male does the most feeding the first two weeks while the female takes care of the nestlings. Unfortunately, the oldest, largest young gets most of the food, and often the second and usually the third young starve unless food is abundant. The parents prefer large fish, which they tear apart for their offspring, but they also haul in carcasses of fish, waterfowl and large mammals. One study found that the diet of nesting bald eagles was 56 percent fish, 28 percent birds, 14 percent mammals and 2 percent other prey. Another study, on the Chesapeake Bay, discovered that Canada geese and mallards were their most common bird prey, and white-tailed deer (presumably carrion) and raccoons their favorite mammal meals. Gulls, ravens, crows, bears and raccoons prey on bald eagle eggs. Nestlings are killed by bears, raccoons, hawks, owls, crows, ravens and bobcats. Siblicide is also common. The fledglings reach their full size in three to four weeks, and fledge anywhere from 8 to 14 weeks of age. Even though they practice beforehand by flapping their wings across their nest and on to nearby limbs to strengthen their muscles, develop flight coordination and learn how to land, more than half the time they end up on the ground where predators can find them.
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Pennsylvania Game News - February 2012

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