With this genus commences the series of Hawk-Eagles as they have very appropriately been termed. They are birds of more slender build, with smaller bills, longer and more slender tarsi, and longer tail than the true Eagles, and most of them are, in some phase of plumage, partly or wholly white beneath.
This genus has a moderately strong bill, much hooked at the end, and with a prominent festoon to the upper mandible; the nostrils are elliptical and oblique; wing long, 4th quill longest, primaries exceeding secondaries by far more than the length of the tarsus; tail nearly square, more than half the length of the wing; tarsi feathered ; toes long, outer toe (without claw) a little longer than inner; claws large, sharp, well curved; inner and hind claw both large, the latter the larger.
The type of Nisaetus, Hodgs., the name used by Jerdon, Sharpe, and others for this genus, was Spizaetus nipalensis, and the generic description does not apply to the present group, although Bonelli's Eagle was included by Hodgson (Ibis, 1894, p. 288). The type of Hieraetus is H. pennatus, the Booted Eagle; but I follow Sharpe in placing the Booted and Bonelli's Eagles in the same genus. Four species are known, of which two are Palaearctic and Indian, one African, and one Australian.
Key to the Species.
a, Larger: wing 19-21………………………H. fasciatus, p. 343.
b. Smaller: wing 14-16.5………………………H. pennatus, p. 344.