This subfamily appears to be composed of two definable sections, with subdivisions as follows:—
A. Tail forked; spinous points of the feathers not extending far beyond the webs, or entirely wanting. Feathering of the sides of the forehead extending forward outside the nostrils nearly to their anterior end.
a. No trace of spinous points to tail-feathers. Feathering of frontal points almost completely enclosing the nostrils between them. No light collar round the neck; sides of the forehead with a hoary suffusion … Nephœcetes.
b. Spinous points of tail-feathers distinct. A light collar round the neck.
1. Nostril as in Nephœcetes. Sides of forehead with a hoary suffusion. Collar chestnut. Wing less than 5.50. (C. rutila.).
2. Feathered frontal points narrower, not reaching anterior end of nostril. Sides of forehead without hoary suffusion. Collar white. Wing more than 8.00 … Hemiprocne.
B. Tail rounded; spinous points of the feathers much elongated and projecting. Feathering of sides of forehead scarcely reaching beyond posterior end of nostril. Wing less than 5.50 … Chætura.
Nephœcetes, Baird, Birds N. Am. 1858, 142. (Type, Hirundo nigra, Gmel.)
