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Birds. A class of animals comprising all oviparous vertebrates which are clothed with feathers, furnished with a bill, and organized for flight. They have warm blood, and a complete double circulation. They are all bipeds; the body is inclined before their feet, the thighs are directed forward, and the toes elongated, forming a broad supporting base. The head and the neck are more or less prolonged, the latter very flexible, and generally containing twelve or more vertebrae. At present birds are divided into nineteen orders, only a few of which can be considered here. In the first order the foot has three toes before and one behind, all armed with long, strong, crooked, and more or less retractile talons, adapted to seize and lacerate living prey (except in vultures). This structure is associated with a strong, curved, sharp-edged and sharp-pointed beak, often armed with a lateral tooth; a very muscular body, and capability of rapid and long-continued flight. This order is termed Raptores. The second type of foot presents three toes before and one behind, and placed on the same level; slender, flexible, of moderate length, and provided with long, pointed, and slightly curved claws. A foot so constructed is especially adapted for the delicate operations of nest building, and for grasping and perching among the slender branches of trees; hence the order so characterized has been termed Insessores, and, from including the smaller tribes of birds, Passeres. In the third type of foot the hinder toe is raised above the level of the three anterior ones; this lessens the power of perching. The other toes are strong, straight, and terminated by robust obtuse claws, adapted for scratching up the soil, and for running along the ground; the legs are for this purpose very strong and muscular. In this group are found the Gallinae or scratching fowls like the hen and the grouse, and the Columbae or pigeons. The modification by which birds are enabled to wade and seek their food in water along the margins of rivers, lakes, and estuaries is gained simply by elongating the bones of the leg (tibia and metatarsus) which are covered with a naked scaly skin. The three anterior toes are very long and slender with the fourth toe either on a level with the others or raised slightly with them. This group includes the Paludicolae, the cranes and rails; the Herodiones or herons; and the Limicolae or shore birds. Swimming birds or Natatores comprise several groups generally with webbed or flattened toes. These birds have the body protected by a dense covering of feathers, and a thick down next to the skin. The whole organization is especially adapted for aquatic life.