Birds
Eagle
Accipitridae · also called eagles
An eagle is a raptor scaled up until it can kill things its own size. The eyes are built like telescopes, the feet close with hundreds of pounds of force, and the wings are long enough to carry all of that over a mountain range on rising air. There is no single eagle branch on the bird family tree — the word marks a job, done by around 60 large species.
Overview
Eagles are the biggest members of the hawk family Accipitridae, roughly 60 species on every continent except Antarctica. They fall into loose groups — sea and fish eagles, booted eagles, snake eagles, and the giant harpy-type forest eagles — that are not each other’s closest relatives. All hunt live vertebrate prey with the feet and, in most, both sexes look alike with the female larger.
Eyes and feet
An eagle’s retina carries about five times the receptor density of ours and has two foveae per eye, giving four to eight times human acuity — enough to spot a hare from kilometres away. The foot is the weapon: a large eagle grips with several hundred PSI at the talon tips, and the harpy eagle’s rear talon rivals a bear claw. Prey is killed by the puncture and crush of the grip, not by the beak, which is for feeding.
Nests and siblicide
Eagle pairs are long-lived and often reuse one nest for decades, piling on sticks until it weighs tonnes. Most lay two eggs a few days apart, and in many species the older chick kills the younger while the parents look on — an insurance system that wastes the second chick whenever food is merely adequate.
Crash and recovery
Eagles show both sides of raptor conservation. The bald eagle fell to about 400 US pairs by the 1960s under the pesticide DDT, then climbed back above 70,000 pairs once DDT was banned — a textbook recovery. Meanwhile many tropical forest eagles, including the Philippine and harpy, are shrinking with the forest.
Status
Eagles as a group are Not Evaluated. Temperate species are mostly stable or rising; tropical and forest species are the concern, hit by deforestation, shooting, poisoning, and collisions with power lines and turbines.
Eagle: common questions
What is the difference between an eagle and a hawk?
Mostly size and power rather than a clean split on the family tree, since both belong to the hawk family Accipitridae and the boundary is a matter of convention. Eagles are the largest members, with heavier bodies, longer and broader wings, much stronger feet, and in many species feathered legs down to the toes. They tend to take larger prey and to soar over open country or big territories. Hawk is used for a range of smaller and medium raptors, including the woodland-hunting Accipiter hawks and the open-country buteo hawks and buzzards. Some medium raptors sit awkwardly between the labels, and different countries draw the line in different places.
How good is an eagle's eyesight really?
Among the best in the animal kingdom. An eagle's retina is packed with about five times as many photoreceptor cells per unit area as a human's, and it has two foveae, the pits of sharpest vision, where we have one, one looking forward and one angled out to the side. The overall result is visual acuity roughly four to eight times ours, so an eagle can resolve detail at 3 km that we would need to be a few hundred metres away to see. Eagles also see into the near ultraviolet and can track fast movement smoothly. The cost is a large, heavy eye that is almost fixed in the skull, which is why an eagle turns its whole head to look around.
Can an eagle really carry off prey heavier than itself?
It can lift and kill prey around its own weight or somewhat more, but it cannot fly off carrying much beyond roughly a quarter to a half of its body weight, because a bird can only generate so much lift. A golden eagle weighing 4 to 5 kg can kill a young deer or a fox but will feed on the spot rather than carry it. Prey up to about 2 kg can be flown to a perch. Stories of eagles snatching human babies or large dogs are myths, and viral videos claiming to show it have all turned out to be fakes.
Why do eagle chicks kill each other?
It is called siblicide, and in some eagles it is almost the rule. The female lays two eggs several days apart, so the first chick hatches with a head start in size and strength. In species such as the Verreaux's eagle and many sea eagles, that older chick pecks and starves the younger one within the first weeks, and the parents do not intervene. The younger egg seems to act as an insurance policy against the first failing to hatch or dying early. Where food is very abundant, as in some bald eagle populations, both chicks more often survive, which suggests the behaviour is driven by competition for limited food.