Eagles are large, powerfully built birds of prey, with a heavy head and beak. Even the smallest eagles, like the Booted Eagle (Aquila pennata) (which is comparable in size to a Common Buzzard (Buteo buteo) or Red-tailed Hawk (B. jamaicensis)), have relatively longer and more evenly broad wings, and more direct, faster flight – despite the reduced size of aerodynamic feathers. Most eagles are larger than any other raptors apart from some vultures. The smallest species of eagle is the South Nicobar Serpent Eagle (Spilornis klossi), at 450 g (1 lb) and 40 cm (16 in). The largest species are discussed below. Like all birds of prey, eagles have very large hooked beaks for tearing flesh from their prey, strong muscular legs, and powerful talons. The beak is typically heavier than that of most other birds of prey. Eagles’ eyes are extremely powerful, having up to 3.6 times human acuity for the martial eagle, which enables them to spot potential prey from a very long distance. This keen eyesight is primarily attributed to their extremely large pupils which ensure minimal diffraction (scattering) of the incoming light. The female of all species of eagle known is larger than the male.
Eagles normally build their nests, called eyries, in tall trees or on high cliffs. Many species lay two eggs, but the older, larger chick frequently kills its younger sibling once it has hatched. The dominant chick tends to be the female, as they are bigger than the male. The parents take no action to stop the killing.
Due to the size and power of many eagle species, they are ranked at the top of the food chain as apex predators in the avian world. The type of prey varies from genus to genus. The Haliaeetus and Ichthyophaga eagles prefer to capture fish, though the species in the former often capture various animals, especially other water birds, and are powerful kleptoparasites of other birds. The snake and serpent eagles of the genera Circaetus, Terathopius and Spilornis predominantly prey on the great diversity of snakes that are found in the tropics of Africa and Asia. The eagles of the genus Aquila are often the top birds of prey in open habitats, taking almost any medium-sized vertebrate they can catch. Where Aquila eagles are absent, other eagles, such as the buteonine Black-chested Buzzard-Eagle of South America, may assume the position of top raptorial predator in open areas. Many other eagles, including the species-rich Spizaetus genus, live predominantly in woodlands and forest. These eagles often target various arboreal or ground-dwelling mammals and birds, which are often unsuspectingly ambushed in such dense, knotty environments. Hunting techniques differ among the species and genera, with some individual eagles having engaged in quite varied techniques based their environment and prey at any given time. Most eagles grab prey without landing and take flight with it so the prey can be carried to a perch and torn apart. The Bald Eagle is noted for having flown with the heaviest load verified to be carried by any flying bird, since one eagle flew with a 6.8 kg (15 lb) mule deer fawn. However, a few eagles may target prey considerably heavier than themselves; such prey is too heavy to fly with and thus it is either eaten at the site of the kill or taken in pieces back to a perch or nest. Golden and Crowned Eagles have killed ungulates weighing up to 30 kg (66 lb) and a Martial Eagle even killed a 37 kg (82 lb) duiker, 7–8 times heavier than the predating eagle. Authors on birds David Allen Sibley, Pete Dunne and Clay Sutton, described the behavioral difference between hunting eagles and other birds of prey thus (in this case the Bald and Golden Eagles as compared to other North American raptors):
They have at least one singular characteristic. It has been observed that most birds of prey look back over their shoulders before striking prey (or shortly thereafter); predation is after all a two-edged sword. All hawks seem to have this habit, from the smallest kestrel to the largest Ferruginous – but not the Eagles.
Among the eagles are some of the largest birds of prey: only the condors and some of the Old World vultures are markedly larger. It is regularly debated which should be considered the largest species of eagle. They could be measured variously in total length, body mass or wingspan. Different lifestyle needs among various eagles result in variable measurements from species to species. For example, many forest-dwelling eagles, including the very large Harpy and Philippine Eagles, have relatively short wingspans, a feature necessary for being able to maneuver in quick, short bursts through dense forested habitats. On the other hand, eagles in the genus Aquila are found almost strictly in open country, are superlative soarers, and have relatively long wings for their size.
Here are lists of the top five eagles going on weight, length and, lastly, wingspan. Unless otherwise noted via reference, the figures listed are the median reported for each measurement in the guide Raptors of the World (Ferguson-Lees, et al.), in which only measurements that could be personally verified by the authors were listed.