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_.
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_.
_b._ Tarsus long; equal to middle toe and half claw; entirely
bare. Tail considerably forked, about equal to closed wing.
Color green above; white beneath … _Callichelidon._[62]