Bill rather weaker; commissure and culmen nearly straight to near tip. Frontal feathers bristly. Tail nearly even. Throat, rump, and crissum, and usually forehead, rufous; belly white … Petrochelidon.
b. Edge of wing smooth. Tarsus longer than in last; equal to middle toe and half the claw. Nostrils bordered along posterior half by membrane, but not overhung internally. Bill very small. Tail forked. Crissum dusky except in Neochelidon fucata. Various genera and subgenera, none North American, as Atticora, Notiochelidon, Neochelidon, and Pygochelidon.
c. Edge of wing armed with stiff recurved hooks. Tarsus as in preceding (tarsus and toes much as in Pygochelidon). Bill larger and more depressed. Tail emarginate only. Crissum white … Stelgidopteryx.
B. Nostrils lateral; bordered behind and inside, or overhung by membrane, the outer edge of which is straight, and directed either parallel with axis of bill or diverging from it.
a. Tarsus short; about equal to middle toe without claw. Tibial joint feathered; feathers extending along inside of upper end of tarsus.
Tarsus bare at lower end. Lateral claws reaching only to base of middle.
Tail very deeply forked, much longer than closed wings; lateral feathers linear and very narrow at end, twice the length of central. Upper parts and pectoral collar steel-blue; front and throat, sometimes under parts, rufous. Tail-feathers with large spots … Hirundo.
Tail with shallow fork, not exceeding half an inch, shorter than closed wings. Feathers broad. Color blue or green above, with or without white rump; white beneath. Tail-feathers without spots … Tachycineta.
Tarsus with a tuft of feathers at lower end. Lateral claws lengthened, reaching beyond base of middle claw.
Tail slightly forked. Color dull-brown above; beneath white, with brown pectoral collar … Cotyle.