Aquila (bird)

Taxonomy and systematics

The genus Aquila was introduced by the French zoologist Mathurin Jacques Brisson in 1760 with the golden eagle (Aquila chrysaetos) as the type species. Aquila belongs to a close-knit group of "typical" eagles including genera Hieraaetus, Lophaetus, Ictinaetus and Clanga. This group occurs as a clade within the larger group of "booted" eagles (tribe Aquilini or subfamily Aquilinae).

The plumage of the more basal members of the booted-eagle group, such as Spizaetus and Nisaetus, generally has barred underparts in adults, and is distinctly different in juveniles which have plain, pale underparts. In contrast, within the Aquila–Hieraaetus–Lophaetus clade, adults are generally dark, with juveniles more closely resembling the adults. Hieraaetus species have both dark and light (or "pied") morphs, with the latter having light, unbarred under-parts.

Research in molecular genetics found Aquila and Hieraaetus to be polyphyletic. Between 2005 and 2014, the British Ornithologists' Union included both Bonelli's and the booted eagle in Aquila. Also, Clements' Checklist merged all Hieraetus species into Aquila from 2001 to 2009. The current approach is to keep Hieraaetus as a separate genus, with Bonelli's eagle and the African hawk-eagle moved into Aquila and Wahlberg's eagle moved into Hieraaetus.

The spotted eagles greater spotted eagle, lesser spotted eagle, and Indian spotted eagle (previously Aquila clanga, A. pomarina, A. (p.) hastata) are thought to be genetically closer to Ictinaetus and Lophoaetus than to other Aquila species, and may be placed into a separate genus, Clanga.

Members of Aquila (excluding those moved to Clanga and Hieraaetus, but including A. fasciata / spilogaster) share two deletions in the (nuclear) LDH gene, as well as similarities in mitochondrial cyt-B gene sequence, though one of these deletions is reverted in A. chrysaetos.

A more recent molecular study by Catanach et al. (2024), which included ultra-conserved genetic elements, supported the existence of three clades: Hieraaetus (including H. wahlbergii), Aquila (including A. spilogaster and A. fasciata), and Lophaetus–Ictinaetus–Clanga. Within Aquila, two main clades were found: nipalensis–adalberti–heliaca–rapax, and audax–guerneyi–chrysaetos–verreauxii–spilogaster–fasciata, with A. africana diverging in between.