The edges of both mandibles smooth, with a single notch,for the indication of one, in the upper; the hinder part of the tarsus longitudinally bilaminated, the laminae entire and smooth; front of tarsus smooth ; wing with nine primaries, the first and second nearly equal; the longest secondaries reaching to about, the middle of the wing; bill flat, broad and triangular when viewed from above; gape very wide; rectrices twelve; sexes alike; a moult in the Spring only; young similar to the adult but with the feathers pale-edged on the upper plumage ; rictal bristles weak.
The Swallows resemble each other very closely in structure, differing only in the shape of the tail. As this, however, is different in almost every species, it cannot be considered a generic character. They can, however, be divided into four fairly well-defined genera by differences such as the feathering of the tarsus and by their colour and colour-pattern.
Key to Genera.
A. Tarsus and toes feathered…………Delichon, p. 226.
B. Tarsus and toes bare, or with a tuft of feathers only.
a. Upper plumage unglossed brown.
a1. Tail-feathers without white spots…………Riparia, p. 231.
b1 Tail-feathers spotted with white…………Ptyonoprogne, p. 236.
b.Upper plumage mostly glossy blue-black…………Hirundo, p. 239.