Birds are Distributed over every Part of the Globe, being found in the coldest as well as the hottest regions, although some species are restricted to particular countries, whilst others are widely dispersed. At certain seasons of the year many of them change their abodes, and migrate to climates better adapted to their temperaments or modes of life, for a time, than those which they leave. Many of the birds of Britain, directed by an unerring instinct, take their departure from the island before the commencement of winter, and proceed to the more congenial warmth of Africa, to return with the next spring. Various causes are assigned by naturalists for this peculiarity, some attributing it to deficiency of food, others to the want of a secure asylum for the incubation and nourishment of their young, and others again to the necessity of a certain temperature for existence; natural selection may be a probable explanation of the phenomenon of the migration of birds. Their migrations are generally performed in large companies; in the daytime they follow a leader who is occasionally changed; during the night-time many of the tribes send forth a continual cry, to keep themselves together, although it might be thought that the noise which must accompany their flight would be sufficient for that purpose.
The Food of Birds varies, as does the food of quadrupeds, according to the character of the species. Some are altogether carnivorous; others, as for instance many of the web-footed tribes, subsist on fish; others, on insects and worms; and others again on grain and fruit. The extraordinary powers of the gizzard of the gramnivorous birds enable them to comminute their food so as to prepare it for digestion. Their digestive system consists of glands of a simple form, of a single or double ingluvies or crop which receives the food; of the proventriculus, or true digestive cavity; of the gizzard furnished internally with horny ridges by means of which the food is broken up, and a comparatively short intestine and gall bladder. The stones found in the stomachs of birds take the place of teeth, in that they grind down the grain and other hard substances which constitute their food. The stones themselves, being also ground down and separated by the powerful action of the gizzard, are mixed with the food, and doubtless contribute greatly to the health as well as to the nourishment of the birds.
All Birds are Oviparous.—The eggs which the various species produce differ in shape and colour as well as in point of number. They contain protoplasm, the elements of the future young, for the perfecting of which in the incubation a bubble of air is always placed at the large end, between the shell and the inside skin. This air bubble gets larger by absorption through the shell and evaporation of the fluid contents, so that a large air-bubble is the sign of a stale egg. During incubation the shell is dissolved, and goes to form the bones of the chick. To preserve an egg perfectly fresh, and even fit for incubation,
