Falco Linn., Syst. Nat., 10th ed. p. 88 (1758).
Type, Falco subbuteo Linn.
In this genus the bill is stout and strongly toothed inside the hooked tip, often with a blunt festoon behind the tooth; the nostril is circular with a central tubercle ; the tarsus is equal to, or shorter than, the middle toe without the claw; the upper part is plumed in front, whilst the exposed portions are covered by small hexagonal scales ; the toes are very long and covered with transverse scales above, the hind toe being shorter in proportion and very powerful; the claws are sharp, curved and strong; the tail is moderate in length and slightly rounded but not graduated; the wing is long and very pointed, the proportions of the primaries varying somewhat; in the Peregrine Falcon the second quill is longest and the first exceeds the third; in the Laggar and the Lanner the first and third quills are about equal, whilst in Sakers the first is the shorter; the second primary is notched on the inner web and in some species this notch extends to the first also.
The Falcons have been much split up by some writers but I follow Sclater in keeping practically all the species under Falco, though I separate the Kestrels, which seem sufficiently well differentiated, both structurally as well as in habits, to be conveniently kept apart, whilst Erythropus seems to be intermediate between the two. In Falco the sexes are sometimes alike and sometimes dissimilar ; the young go through one or more phases before reaching the fully adult plumage which, in some cases, takes several years to complete.
Key to Species.
A.Size large ; raid-toe without claw over 40 mm.
A.First primary longer than third; upper parts ashy-grey or slaty-grey in adults.
a1. Cheek-stripe broad; no nuchal collar………….F. peregrinus, p. 32.
b1. Cheek-stripe narrow; a buff" nuchal collar...............F. peregrinoides, p. 36.
B.First primary about equal to third; upper parts not ashy- or slaty-grey.
c1.A distinct narrow cheek-stripe; middle tail-feathers entirely brown in adults............. F. jugger, p. 37.
d1. No cheek-stripe; middle tail-feathers with white spots on each web…..….F. cherrug, p. 30.
B.Size small; mid-toe without claws under 40 mm.
c.Second quill longest; first much longer than fourth.
e1. Breast white or buff with brown streaks............... F. subbuteo, p. 41.
f1. Breast deep rufous, unspotted in adults ............... F, severus, p. 45.
d.Second and third quills subequal; first and fourth quills subequal.
e1. Crown grey or brown ............... F. columbarius. p. 49.
d1. Crown chestnut............... F. chiquera, p. 47.