Family HIRUNDINIDÆ.—The Swallows.
Char. Bill short, triangular, very broad at base (nearly as wide as long) and much depressed, narrowing rapidly to a compressed, notched tip; mouth opening nearly to the eyes. Primaries nine, graduating rapidly less from the exterior one; tail-feathers twelve. Feet weak; tarsi scutellate, shorter than middle toe and claw. Number of joints in toes normal; basal joint of middle toe partially or entirely adherent to lateral toes. Wings long, falcate. Tail forked. Eyes small. Plumage compact, usually lustrous. All the American species with a white patch on the sides under the wing, and with the irids hazel or brown.
The Hirundinidæ form a very well marked group of birds easily distinguished from all others. They exhibit a close resemblance, in external appearance and habits, to the Cypselidæ; from which, apart from the internal structure, they are readily distinguished by the possession of nine, instead of ten primaries; twelve, instead of ten tail-feathers; scutellate tarsi, toes with normal number of joints (1, 2, 3, and 4, respectively, exclusive of ungual phalanges), instead of a different proportion; differently shaped nostrils, etc. In both families the wings are developed to an extraordinary degree; the outer primary nearly twice or more than twice the length of the inner, and enabling its possessor to sustain flight almost indefinitely. The relations of the family among the Oscines appear closest to the Old World Muscicapidæ.
In comparing the wings of the Hirundinidæ with those of the Cypselidæ we readily notice one of the essential characters of the Oscines, namely, that the greater wing-coverts hide only half or less than half of the secondary quills, instead of reaching much beyond their middle, or nearly to the end. (See Sundevall, Ornith. Syst.)
The precise character of scutellation of tarsus is somewhat difficult to make out, owing to a tendency to fusion of the plates, although not essentially different from most Oscines. There is a series of scutellæ along the anterior face of the tarsus, and a longitudinal plate on each side, meeting, but not coalescing, behind. The anterior scutellæ sometimes appear to fuse into the outer lateral plate; or sometimes the latter is more or less subdivided; the inner plate is generally more distinct from the anterior scutellæ, and usually entire, except perhaps at the lower extremity.
Genera of North American Hirundinidæ.
A. Nostrils broadly oval, or circular; opening upwards and forward, and exposed; without overhanging membrane.
a. Edge of wing smooth. Tarsus short, stout; equal to middle toe without claw; feathered on the inner side above. Nostrils almost or entirely without membrane.
Bill stout; culmen and commissure much curved. Frontal feathers without bristles. Tail deeply forked. Color lustrous-black; belly and crissum sometimes white … Progne.