Head of moderate size, oblong; neck of moderate length; body very slender. Feet very small; tibia bare for a considerable space; tarsus very short, slender, compressed, covered anteriorly with twenty-two small scutella, laterally and behind with reticular scales; toes very small, slender, the first extremely small, the third longest, the fourth considerably shorter, the second shorter than the fourth in the same proportion; the anterior toes connected by reticulated webs, which are deeply concave at their margin. Claws arched, compressed, that of the hind toe smallest, of the middle by much the largest, and having the inner edge thin and dilated.
Plumage soft, close, blended, very short on the fore part of the head; the feathers, in general, broad and rounded; wings very long, narrow, and pointed; primary quills tapering to a rounded point, slightly curved inwards, the first longest, the rest rapidly graduated; secondary quills short, broad incurved, obliquely rounded, the inner more tapering. Tail long, very deeply forked, of twelve feathers, of which the outer are tapering, the middle short and rounded.
Bill bright coral-red, black towards the end, the tip light yellow; inside of mouth reddish-orange; eye hazel. Feet coral-red, lighter than the bill; claws brownish-black. Upper part of the head, and the hind neck half-way down, deep black, the anterior part tinged with brown, the posterior with blue. The sides of the head, the fore neck, and all the lower parts, white, with a slight tinge of greyish-blue on the breast. Back, scapulars, and wings, light greyish-blue, the edges of the wings, the rump, and upper tail-coverts, white, slightly tinged with grey. First primary, with the outer web deep black, the shaft white, on the inner web a greyish-black band running along the shaft, narrow at the base, and widening so as to occupy the whole breadth of the web for an inch at the end, where it is hoary. The next five have the outer web, and a varying portion of the inner, in nearly their whole length hoary, but at the same time with a dusky shade, which becomes more apparent at the ends; the rest of the quills are like the back, but margined and tipped with white. Tail-feathers with the inner webs white, the outer webs of the colour of the back, paler on the middle feathers, gradually deepening outwards, and on the outer feathers dark or blackish-grey.
Length to end of tail 16 inches, to the fork of the tail 11, to end of wings 15 3/8, to end of claws 11 1/4; extent of wings 31 1/2; wing from flexure 11 5/12; tail to end of lateral feathers 7 1/12, to fork 3 1/12; bare part of tibia 6 1/2/12; tarsus 10 1/2/12; hind toe and claw 3 1/2/12, middle toe and claw 1 1/12. Weight 5 oz.
The female is similar to the male, but rather smaller. In some instances I have seen a small portion of the forehead white.
Length to end of tail 15 inches, to the fork 11 1/2, to end of wings 15 1/4, to end of claws 11; extent of wings 30 1/4; wing from flexure 10 1/2. Weight 5 oz.
The young in their first plumage, have the bill dull greenish-black, with the tip yellowish; the feet greenish-yellow.
In winter, the bill is black, with the base pale orange, and the tip yellowish; the feet orange-yellow. The colours are as in the adult, the forehead white, the rest of the head dusky, the upper parts having the feathers slightly margined with lighter.
Length to end of tail 12 3/4, to the fork 11; to end of wings 14, to end of claws 10 1/2; extent of wings 29 1/4; wing from flexure 8 1/4.
American and British specimens present no essential differences when compared in considerable numbers. The outer web of the lateral tail-feather is blackish-grey, and the inner webs of the tail-feathers are white in all the specimens collected for comparison. The tarsus in American specimens varies in length from 9 to 10 1/2 twelfths, and the claw of the middle toe from 2 1/2 to 4 1/2 twelfths; but similar differences are observed in the British birds.