Aquilinae

Life histories and range

Booted eagles are varying in their habitats and habits, being found on every continent inhabited by accipitrids, which includes all continents with the exception of Antarctica. They may inhabit nearly all of the world's terrestrial habitats, with the majority of species being largely forest dwelling, but several preferring all kinds of open habitat from steppe to prairie to tundra as well as most rocky or mountainous areas. By far the widest ranging species of booted eagle is the golden eagle, which is distributed in most of North America (where it is the only species of the subfamily north of Mexico) and much of Eurasia including a majority of Europe, often along most mountainous terrains therein but also other land-based habitats with typically remote semi-open or hilly hunting grounds accessible. So-called "hawk-eagles" are forest dwelling booted eagle types largely represented by Spizaetus (in the neotropics) and Nisaetus (in Asia). Despite many similar features in the two main hawk-eagle genera, they apparently evolved separately for some time and are not closely related. The peak diversity for Aquilinae are in Africa, where nearly half of extant species are known to dwell at least seasonally, exploiting a diversity of different hunting styles, habitats, nesting habits and general body forms. All booted eagle species are opportunistic predators which prey mostly on small vertebrates, often subsisting on a wide range of mammals, birds, reptiles and amphibians as they become locally available or abundant (carrion is also rarely ignored). Species favoring more open habitats tend to hunt most often on the wing, using distance or hugging contours of the ground to surprise prey. Forest dwelling species are more likely to be perch-hunters, watching for prey activity from a high tree perch, but every kind of booted eagle can vary their hunting techniques when necessary. In cases where they become specialists, booted eagles tend to be mammal hunters, such as the considerable dependence of Spanish imperial eagle (Aquila adalberti) on rabbits, of breeding steppe eagle (Aquila nipalensis) on sousliks or Verreaux's eagle (Aquila verreauxii) on hyraxes. Exceptions include the Ayres's hawk-eagle (Hieraaetus ayresii) and, to a lesser extent, the rufous-bellied eagle (Lophotriorchis kienerii), both of which are smallish, swift-flying bird-hunting specialists who dive (falcon-like) on woodland birds while the black eagle (Ictinaetus malaiensis) feeds largely on the contents of birds' nests and squirrel dreys (the only hawk or owl specialized to do so). Most booted eagle species have relatively large feet and talons and can semi-regularly go after prey as large or larger than themselves, even smaller species can occasionally dispatch prey of up to two to three times their own weight. Rare attacks on large prey, such as ungulates, adult monkeys or medium-sized carnivores or other much larger animals, usually involve the larger species of booted eagle, whereas such attacks are exceptionally rare to non-existent in eagles of other lineages. At least four large booted eagles have been known to dispatch prey weighing 30 kg (66 lb) or more.

Booted eagles, like almost all raptorial birds, are strongly territorial and tend to maintain expansive home ranges against conspecifics. Given their need for ample hunting ranges, the territory of most booted eagles tend to be extremely large, with several dozen square kilometers being common. Territories are usually maintained with display flights but vocal displays are known in some of the denser forest dwelling varieties. Nests of Aquilinae tend to be typically large as in most eagles, with ample surface space needed, whether located in trees, rocky formations or on the ground. As is typical in many raptorial birds, pair bonds are strong between males and females and in many booted eagle species, they may mate for life. Primarily females incubate the egg and brood the young, while males usually have the responsibility of food capture. Clutch sizes are usually small in booted eagles, rarely more than 3 eggs are laid, and most parents will manage to produce only one to two fledglings depending on species. In most booted eagles, beyond threats from the outside world, most eggs are laid and hatched at intervals, thus one sibling is usually considerably bigger and often kills its younger siblings. While brood size and fledgling rates are typically low, booted eagles may have even lower productivity due to the long dependence period of the young relying on their parents for food and protection, and some species may only nest every other year.

General status

Due to their large territories and low productivity, most booted eagle species are sparsely distributed and not infrequently uncommon-to-rare even in regional strongholds where ample habitat remains. Booted eagle species tend to be highly sensitive to human activities, mainly habitat alteration or destruction, human disturbance, collision with man made objects (especially non-insulated electrical pylons and wind turbines), accidental or intentional killing of staple prey species and various forms of persecution by humans. Due mainly to these factors, seven species of booted eagle are currently classified as Vulnerable to extinction by the IUCN. Four little-known forest dwelling species are classified in more severe status as Endangered: the Philippine hawk-eagle (Nisaetus philippensis), Pinsker's hawk-eagle (Nisaetus pinskeri), Javan hawk-eagle (Nisaetus bartelsi) and the black-and-chestnut eagle (Spizaetus isidori). A very different eagle, the steppe eagle, is also considered Endangered despite having been considered one of the most numerous of all eagles after a disastrous, ongoing decline mostly due to electrocutions from dangerous powerlines, poisonings and increasing steppe fires around nests, these killing them off en masse while breeding and migrating. An even more drastic classification of Critically Endangered was given to the Flores hawk-eagle (N. floris). In all five endangered hawk-eagles, near epidemic levels of forest habitat degradation, primarily direct deforestation, has been determined as the primary cause of their declines.

Systematics

The cladogram of the Aquilinae shown below is based on a molecular phylogenetic study of the Accipitridae by Therese Catanach and collaborators that was published in 2024.