An eagle pair typically returns to the same territory and nest site each breeding season. This territory may cover an area greater than a square mile and include more than one nest. From year to year, the pair may abandon a nest for unknown reasons and build another, or it may continue adding to and reno-vating a single nest. These second or third nests are alternative nests, but they may return to their original nest at some point.
In addition to large stands of trees, the nesting habitat must provide an abundant food source. Ample prey in the form of medium and large fish will supply a nesting pair of eagles with the needed food to sustain themselves and their growing eaglets through the nesting season.
Eagles defend their territory against potential nest predators like hawks, owls, gulls, crows, and ravens. They also protect the territory from other eagles, especially adult eagles. However, nest predators may include mammals such as raccoons, fishers, and bears that can hunt at night when eagles are vulnerable.
Viewing Tip: Repeated sightings of two adult eagles together in an area during late winter could be a sign that the area is part of a nesting territory.
Nest Tree and Nest
Because eagles build sizable nests, a nest tree must be large and sturdy. Eagle nests are usually within view of a lake, river or large creek and a pair often chooses a dominant tree within the surrounding woodland. A good nest tree affords an easy glide upon leaving the nest and provides a favorable vantage point to scout for potential threats. Some nests are located near human activity or roads, but do not get approached regularly or directly by people.
Eagles build their nest in a branched crotch toward the top of the tree. The birds stack and interweave sticks and branches to create a bulky nest and line its center with soft material such as moss, grass, twigs and feathers. If the nest tree remains intact through inclement weather, natural aging and other environmental influences, over the years a nest may reach enormous dimensions. A nest may exceed eight feet wide, 12 feet high and weigh more than two tons. Typical nests are five to six feet wide and more than three feet high. The bald eagle builds the largest nest of any North American bird. Sometimes nests are so large — compared to their support tree — they collapse the tree, especially when heavy snow or rains augment their weight. When trees decline in health sometimes they are no longer capable of supporting a large, heavy eagle nest. So, eagles may be forced to built another nest in another large tree.
Nest building and renovation may begin as early as November or December in areas where eagles remain local through winter. Some pairs need to rebuild their nest almost from scratch if winds and bad weather have damaged it. This nesting activity starts one to three months before the female lays eggs. This nesting activity tends to occur later in areas where the snow and ice linger at higher elevations or in areas sheltered from the sun.
Viewing Tip: An eagle nest is usually 40 to 100 feet above the ground. In Pennsylvania, eagles favor white pine, sycamore, red oak, red maple and tulip poplar for nesting.
Acts of courtship or renewing pair bonds can be as subtle as two eagles perching together on a branch or the pair rearranging twigs and dried grass at the nest. Courtship also can include spectacular flight displays such as high-speed aerial pursuits, the pair rising high on a thermal and soaring together, whirling and tumbling in flight and then — most impressively — plummeting toward the ground with locked talons and splitting just seconds before impact.
The eagle pair also vocalizes back and forth with a variety of shrill calls. Courtship behavior often begins in winter.
Viewing Tip: Elaborate courtship displays are observed less often than the subtle signs of pair bonding.
Eggs in the Nest
Incubating begins as the female lays the first egg. The clutch of one to three eggs is completed within three to six days. Both parents share in the task of incubating. However, females spend more time brooding than males. Once they begin incubating, the viewer usually sees only one eagle at a time, except when they make the incubation duty changeover at the nest. In only one case has there been four eggs documented in a Pennsylvania nest, a rare event nationwide. That Delaware River nesting pair fledged all four eaglets in Northampton County in 2009.
Each egg takes 35 days to hatch. Eggs must be kept warm, shaded from harsh sunlight and protected from predators.
Viewing Tip: An eagle sits very low in the nest when on eggs. Sometimes only its head and tail are visible. One eagle will bring food to the bird on duty and the two may switch roles. The incubating bird will carefully position itself by sitting chest first while wiggling side to side to accommodate the egg or eggs. A bird on eggs sits remarkably still for long periods. This is the period when nests are most vulnerable to abandonment. When eagles are flushed from their nest, their eggs may be exposed to cold or rainy weather and predators. Cold air can cause egg failure. Eagles may abandon their nest if they are flushed repeatedly during this period.
Young in the Nest
Young eagles break through and escape the egg-shell on their own. Eggs hatch one at a time with one to four days between hatchings. Eaglets grow rapidly giving the "oldest" a competitive advantage over nestlings hatched only a day or so later, especially when food is short.
The adult on the nest calls to its mate for food when the foraging eagle is near. The birds communicate back and forth as the eagle bringing food approaches the nest. Both adults tear the fish apart and feed the young. The demand for food increases with the growing eaglets, there-fore, adults come and go frequently to share in foraging and tending to the young birds.
Viewing Tip: As eggs hatch, behavior changes at the nest. An eagle sitting on hatchlings is more active than when on eggs. The brooding bird repositions itself often and frequently looks down to check the young.
Eagles learn and hone their ability to fly several weeks before actually flying. They practice with short takeoffs and landings on and around the nest, gaining strength and improving their agility and landing ability. This is a crucial stage for the young birds as many try to fly prematurely, especially when startled or flushed from the tree by predators and people who get too close. If its first flight is unsuccessful, a young eagle may spend time on the ground exposed to predators and other hazards.
When about 12 weeks old, the young eagles are ready to fledge. Parents continue to deliver fish and other prey to the young birds until they learn to forage on their own, which occurs between four and 10 weeks after leaving the nest. This stage is a very vocal time for the eagles as the adults and young call frequently to locate each other for feeding and security. The eagles usually remain near the nest area through much of summer.
Viewing Tip: As they grow, young eagles gradually become more visible on the nest. By the time they're eight weeks old, it is common to see the nestlings perched on branches around the nest. After they leave the nest, it takes practice for them to become proficient at flying. Landings are almost always awkward and somewhat conspicuous for newly fledged eagles.