Birds.--Birds among all other animals are known by their covering of feathers and the presence of wings. The feathers are developed from the skin. These aid in flight, and protect the body from the cold. [Illustration: Adaptations in the bills of birds. Could we tell anything about the food of a bird from its bill? Do these birds all get their food in the same manner? Do they all eat the same kind of food?] The form of the bill in particular shows adaptation to a wonderful degree. A duck has a flat bill for pushing through the mud and straining out the food; a bird of prey has a curved or hooked beak for tearing; the woodpecker has a sharp, straight bill for piercing the bark of trees in search of the insect larvae which are hidden underneath. Birds do not have teeth. [Illustration: Common tern and young, showing nesting and feeding habits. (From group at American Museum of Natural History.)] The rate of respiration, of heartbeat, and the body temperature are all higher in the bird than in man. Man breathes from twelve to fourteen times per minute. Birds breathe from twenty to sixty times a minute. Because of the increased activity of a bird, there comes a necessity for a greater and more rapid supply of oxygen, an increased blood supply to carry the material to be used up in the release of energy, and a means of rapid excretion of the wastes resulting from the process of oxidation. Birds are large eaters, and the digestive tract is fitted to digest the food quickly, by having a large crop in which food may be stored in a much softened condition. As soon as the food is part of the blood, it may be sent rapidly to the places where it is needed, by means of the large four-chambered heart and large blood vessels. The high temperature of the bird is a direct result of this rapid oxidation; furthermore, the feathers and the oily skin form an insulation which does not readily permit of the escape of heat. This insulating cover is of much use to the bird in its flights at high altitudes, where the temperature is often very low. Birds lay eggs and usually care for their young. [Illustration: African ostrich, one of the largest living birds.] CLASSIFICATION OF BIRDS ORDER I. _Cursores._ Running birds with no keeled breastbone. Examples: ostrich, cassowary. ORDER II. _Passeres._ Perching birds; three toes in front, one behind. Over one half of all species of birds are included in this order. Examples: sparrow, thrush, swallow. ORDER III. _Gallinae._ Strong legs; feet adapted to scratching. Beak stout. Examples: jungle fowl, grouse, quail, domestic fowl. ORDER IV. _Raptores._ Birds of prey. Hooked beak. Strong claws. Examples: eagle, hawk, owl. ORDER V. _Grallatores._ Waders. Long neck, beak, and legs. Examples: snipe, crane, heron. ORDER VI. _Natatores._ Divers and swimmers. Legs short, toes webbed. Examples: gull, duck, albatross. ORDER VII. _Columbinae._ Like Gallinae, but with weaker legs. Examples: dove, pigeon.