The young are fed on a substance called ‘pigeon’s milk,’ a thick white, curd-like fluid, consisting of the partially digested food the parent bird has swallowed, and which is regurgitated from its crop. In feeding, the young bird thrusts its beak deep down into the mouth of its parent and literally drinks. The pigeons alone among birds feed their young in this way; and they also differ from other birds in drinking like mammals, taking a continuous draught instead of a series of sips. Stock-Dove. Columba œnas. Head, throat, wings, and lower parts bluish grey; the lower parts of the neck with metallic reflections; breast wine-red; a black spot on the last two secondaries and some of the wing-coverts; primaries grey at the base, passing into dusky; tail grey, barred with black at the extremity, the outer feather with a white spot on the outer web, near the base; iris reddish brown; bill yellow, red at the base; feet red. Length, thirteen and a half inches. * * * * * The stock-dove is a third smaller than the wood-pigeon, and in size, colouring, and appearance when flying, so closely resembles the common pigeon, or rock-dove, as to be often mistaken for it. But it differs from the better-known bird in the uniform blue colour of the back: the rock-dove has a white patch on the rump. It is not so abundant nor so widely diffused as the species last described, being most common in the southern and eastern counties of England; but it is found in suitable localities throughout England and Wales, and is extending its range in Scotland; also, in a less degree, in Ireland. In some localities in the south it is so abundant that its low, monotonous, crooning or ‘grunting’ voice may be heard all day long in summer like a continuous murmur in the woods. It prefers ancient woods, and breeds in holes in trees and pollard tops, and from this habit it is said to derive its name of stock-dove. It is also an inhabitant of seaside cliffs, like the rock-dove; and at Flamborough Head, on the Yorkshire coast, both species may be found breeding in the same caverns, and sometimes associating in flocks together. In districts with a sandy soil it nests on the ground in a rabbit-burrow, or under a thick furze-bush. A very slight nest is made of twigs and sticks, and in many cases no nest at all. The eggs are two in number, and of a light cream-colour. Rock-Dove. Columba livia. Bluish ash, lighter on the wings; rump white; neck and breast lustrous, with green and purple reflections; two transverse black bands on the wing; primaries and tail tipped with black; outer tail-feathers white on the outer web; iris pale orange; bill black; feet red. Length, twelve and a half inches. * * * * * [Illustration: FIG. 88.--ROCK-DOVE. ⅐ natural size.]