Bald Eagle

Eagles have eyesight estimated at 4 to 8 times sharper than a human’s, reputedly letting them spot a rabbit from about 3.2 km (2 mi) away. Eagles in general are credited with roughly 20/5 vision, compared with 20/20 for an average person, meaning an eagle can see at 6 m (20 ft) what a human must stand about 1.5 m (5 ft) from to make out. The secret lies in the retina, where a pit called the fovea packs about one million cells per square millimeter, compared with roughly 200,000 in humans. Eagles also have two foveae in each eye, which together support sharp, high-resolution vision and fine detail. Bald eagles share this remarkable vision, which helps explain how a perched bald eagle can pick out a fish near the water’s surface from far away.

Bald eagles usually catch fish by swooping low and snatching them from the surface or just below it with their talons, rather than diving in fully like an osprey, which plunges feet first about 30 to 60 cm (1 to 2 ft) beneath the water. Eagles seldom dive vertically, relying on a shallow approach instead. This technique helps an eagle keep most of its feathers dry. Spiny structures on the toes help it grip slippery prey. Things can go wrong, though: if an eagle misses or grabs a fish too heavy to lift, it may end up in the water, where soaked feathers keep it from flying. Eagles can then swim to shore by pushing down with their wingtips in a stroke resembling a butterfly stroke, but swimming takes enormous energy, and some eagles drown.

Their nests, called eyries, are reused and enlarged over many years, sometimes growing to weigh about 1 metric ton (2,200 lb) as pairs keep adding material. A bald eagle builds the largest nest of any North American bird, and an old, much-expanded nest can reach roughly 4 m (13 ft) deep and 2.5 m (8.2 ft) wide. A typical nest is about 1.5 to 1.8 m (5 to 6 ft) across and 0.6 to 1.2 m (2 to 4 ft) tall, built near the trunk high in the tree but below the crown. Building can take up to three months, with both partners bringing materials while the female does most of the placement. The largest tree nest ever recorded for any animal, in Florida, was 6.1 m (20 ft) deep and 2.9 m (9.5 ft) wide.

Bald eagles generally mate for life and reinforce their bond through spectacular aerial courtship displays. Males perform calls and flight displays that include swoops, chases, and cartwheels. The most dramatic is the cartwheel: a pair flies high, locks talons, and tumbles toward the ground in a spinning free fall, breaking apart at the last instant. The maneuver is risky, since the pair must release their locked talons in time to avoid hitting the ground. High mate fidelity helps a pair work together season after season, but the bond is not unbreakable. If one partner dies or disappears, the survivor chooses a new mate, and pairs that repeatedly fail to breed may split up. Even so, pairs often reuse and add to the same large nest over many years, renewing both their territory and their partnership.

Both parents share incubation and chick care throughout the breeding season. They take turns sitting on the one to three eggs for about 35 days, although the female does most of the sitting, while the other parent hunts or gathers nesting material. For the first two to three weeks after hatching, an adult is almost always at the nest. After five to six weeks, the parents usually spend much less time there, often perching in nearby trees. Young eagles fledge at 8 to 14 weeks old, and their parents keep caring for them for about six more weeks. Juveniles begin dispersing roughly eight weeks after fledging, finally leaving the family territory to start their own lives. With only one brood a year, this careful teamwork is essential to getting young eagles through to full independence.

Eagles are known to steal fish from ospreys, harassing the smaller raptor until it drops its catch. Scientists call this behavior kleptoparasitism. An eagle will often perch and watch for an osprey to make a catch, then harass it until it drops the fish. Ospreys do the hard work, plunging feet first into the water, sometimes 30 to 60 cm (1 to 2 ft) deep, while the eagle takes the reward. Eagles also steal from other animals, including herons and even otters, and they frequently harass other eagles for food. The behavior is more common in winter, when fish are harder to find. Stealing lets an eagle save the energy it would spend hunting. Though eagles can catch fish themselves, they are opportunistic feeders, taking whatever meal is easiest to get.