SENSES of sight, smell, and hearing are remarkably acute in birds. This is especially true of sight. Some have three eye lids, the upper and lower and a membrane which can be drawn over the entire eye-ball, called the nictitating membrane, enabling them to look directly at the sun. Eagles, Hawks, and Owls are thus provided. The eyes of the Eagles and Hawks are provided also with a ring of bony plates, by means of which the eye adjusts itself like a telescope, taking in both near and far objects.

Birds, except nocturnal species, have no external ear, but hear well.

The young of nest building birds when born are blind, naked, and unable to walk. In the Hen, the Partridge, and the Ducks, the young are able to walk, swim, or pick up food, as soon as they break the shell. When moulting, at the close of the breeding season, Quails usually shed in pairs. The male generally assumes a duller hued coat than the female.

With most birds of prey the female is the larger.

Bird's songs are composed of love-notes and pleasure-notes. We speak of the scream of birds, their chirp, expressions of joy or fear, as in the human voice. Their songs can be set to music. See S. P. Cheney's "Wood Notes Wild," in which the songs of many of our common birds are thus reproduced. The odd and peculiar actions of birds, their dances, struts, and posturings are all expressions of their emotions.

The nesting habits of birds are varied. Gulls drop eggs on bare ground or rocks; the Baltimore Oriole and Tailor bird construct hanging nests of elaborate workmanship; the Woodpecker hews out a deep nest in a rotten limb; the Kingfisher digs one out of a sandy bank, while the Cuckoo takes possession of the nest of some other bird. Most birds select nesting places away from other species, but Swallows, English Sparrows, Grackles, and Crows live in communities.

There are between seven and eight thousand species of living birds. A few species have become extinct, specimens of which it is the intention of Birds to present in future numbers.

There are three centers of distribution in the United States: (1) the Atlantic states and Mississippi valley; (2) the Rocky Mountain plateau; (3) the Pacific coast.

Most of the birds breeding in the northern portions of the United States migrate south during the winter months. Those remaining are known as residents.