Eagles: behavior & cognition

Eagles are opportunistic foragers whose methods depend heavily on species and habitat. Bald eagles take fish as a staple but also hunt waterfowl and small mammals, and they scavenge carrion and refuse readily — a flexibility that lets them exploit seasonal gluts such as spawning salmon or winter waterfowl die-offs. Golden eagles, by contrast, are documented hunting live mammalian prey such as rabbits and hares, attacking both from the air and on the ground. The Cornell Lab notes that golden eagle pairs sometimes hunt jackrabbits cooperatively during the breeding season, with one bird stooping to divert the prey while its mate makes the strike.

A well-recorded behavior is kleptoparasitism, or food piracy. Bald eagles are widely described harassing ospreys and other birds until the victim drops or releases its catch, which the eagle then seizes in the air or off the water. The role of piracy can rise sharply when other food is scarce: a study of bald eagles wintering along Nebraska's North Platte and Platte rivers in 1978–1980 documented them stealing prey mainly from other raptors — hawks and other eagles — and found that stealing occurred far more often during the severe winter when ice cover cut the birds off from fishing. That study also reported subadults watching adults steal and joining in, which the authors said suggested the tactic may be learned — an observational inference rather than an experimentally demonstrated mechanism.

Many eagles hold and defend nesting territories, particularly during the breeding season. Animal Diversity Web describes wedge-tailed eagles (Aquila audax) becoming territorial in the breeding season and defending the nest and surrounding area from other eagles of their kind. Golden eagle pairs likewise maintain large nesting territories and are described performing conspicuous aerial displays — high circling, steep dives, and a rolling "undulating" flight — that tend to occur at territory edges or when a territorial bird encounters an intruder, functioning as advertisement and boundary signaling rather than necessarily ending in physical combat.

Territory size and the intensity of defense vary with species, prey density, and habitat, so a single figure does not describe all eagles. Holding a large territory is tied to the considerable spacing eagles need to find enough food and suitable nest sites; where resources are rich, such as productive coastlines, breeding pairs may nest comparatively closer together. Defense is generally directed most strongly at conspecifics — birds of the same species competing for the same nest sites — rather than at every passing animal.

Eagles invest heavily in large, reused nests (eyries). Pairs often build durable stick platforms and refurbish them across many seasons; the bald eagle is famous for some of the largest tree nests of any bird, with a record nest near St. Petersburg, Florida measured at about 2.9 m (9.5 ft) across — though such giants are exceptional rather than typical. Care is usually shared but unevenly divided: in golden eagles the female does most of the incubation (with the male assisting) and broods the young closely in their early weeks, while in African fish eagles (Haliaeetus vocifer) the female primarily incubates and shades the chicks and the male does most of the hunting. Bald eagle eggs hatch after roughly 35 days, and chicks remain in the nest for about 10–13 weeks before fledging.

A sobering aspect of eagle parenting is brood reduction through sibling aggression, sometimes called cainism. Because eggs hatch a day or more apart, the older chick is larger and may attack the younger one. In golden eagles this aggression is common and frequently fatal to the second chick; in bald eagles it is rarer and depends more on food supply. Researchers understand the behavior as an evolved form of brood reduction in resource-limited environments rather than as cruelty, and parents typically do not intervene.

Behavior claims here are drawn cautiously from institution-backed references and described with their evidence context and limits. See animal research sources for the methodology, the behavior cluster hub for the wider topic, and animal senses & adaptations for the underlying biology.