Among birds adapted to life in the water the penguins are pre eminent. These include about 17 species found in southern seas, ranging in size from the great emperor penguin (Aptenodytes forsteri), which is 48 in. long and weighs up to 78 lb., to the little blue penguin (Eudyptula minor) of New Zealand and the Chatham islands, which is only 16 in. in length. Penguins have thick, heavy bodies, more or less elongated, and are covered uniformly with dense, short feathers, without the bare spaces or apteria that in other birds divide the feather-growths into distinct tracts. The wing is a short, broad paddle, without de veloped quills, the feathers being close-set and stiff, almost like the scales of a reptile. In the water the penguin rests com paratively low. Beneath the surface it progresses literally by fly ing, the flattened wings driving the bird through its aquatic medium swiftly and gracefully, while the feet are extended behind and serve as a rudder. Penguins have possessed their present type of body since early Tertiary times; but they came originally from a flying stock.
Other types of birds developed for life in the water include loons or divers and grebes, whose progress beneath the surface is accomplished, usually, by the use of the broad feet alone, the wings being held close to the sides, except when the birds are frightened or under other unusual circumstances. The downy young dive, by use of both wings and feet, indicating that flying under water is a primitive method. The many species of auks and guillemots use the wings beneath the surface as in the air above. The cormorant and the snake-bird progress by use of feet alone. Mergansers, scaups, redheads, pochards and golden-eyes among ducks dive regularly with the feet alone, while in the same group old squaws, scoters and eiders as regularly use both wings and feet beneath the surface. The curious diving petrels (Pele canoides) of the southern hemisphere fly swiftly beneath the water, and may burst out through the surface in full flight.
Birds adapted for life at night are fairly numerous, and are typified especially by members of the nightjar and owl families, though specialized forms occur among a number of other families. Most nocturnal birds have large eyes that usually reflect light with a prominent reddish colour so that the eyes of many night jars glow like dull coals of fire by the reflected light of an electric torch, and may be seen for a considerable distance. One of the American night-hawks (Chordeiles acutipennis) is an exception, as the eye is said to shine with a pale green hue. Most nocturnal birds actually see at night by a specialized eye adapted to collect the faintest rays of light. Only the kiwis (Apteryx) among habitually nocturnal species appear wholly blind.
The laws governing geographic distribution (see DISTRIBUTION OF ANIMALS ; ZOOLOGICAL DISTRIBUTION), through which each faunal area has its own peculiar forms, apply to birds as to all other animals, in spite of the easy method of travel possible for the bird. Very few birds are cosmopolitan. As an example may be mentioned the sanderling (Crocethia alba) that at some season may be found along water almost anywhere on the Earth, but that nests only in the Arctic regions, passing southward in migra tion throughout the world, to return in spring to its breeding area. The barn owl (Tyto), resident through wide areas in the temper ate and tropical regions of the earth so that it reaches all the continents, has reacted to its environment in such a way that 25 geographic forms, some sufficiently distinct to be called species, are now recognized. In contrast to this, many species are confined within very narrow limits, as the very distinct Laysan teal (Anas laysanensis), found only about the shores of one small lagoon in Laysan islands in the Hawaiian Bird Reservation, where it has a range of only about 1 sq. mile.
Every continent has its peculiar forms of life, so that the earth's surface has been divided into great regions, each charac terized by certain groups or by the lack of some found elsewhere. According to a usually accepted classification, these are given the following names : Nearctic region, for North America south to include the Mexican tableland; Neotropical region, for southern Mexico, Central and South America, and the West Indies; Palae arctic region for Europe, Algeria and Morocco, and Asia north of the Himalayas and the Gobi desert ; Ethiopian region, for the remainder of Africa; Oriental region, for India, Indo-China, China, the Philippine islands, and the East Indian islands to, and includ ing, Bali; Australian region, for Australia, and the Pacific islands south of the Tropic of Cancer ; and New Zealand region, for New Zealand. The life of the northern hemisphere is so evidently allied that what are here designated Nearctic and Palaearctic regions are frequently united under the term Holarctic, a vast area that extends throughout the entire north temperate and Arctic area.
These great regions, which cover continents, are divided into life-zones, where temperature and certain other general conditions control the distribution of species within narrower limits. The life-zones are more sharply delimited in temperate regions than in tropical and sub-tropical areas, and to some extent are more easily distinguished in the northern hemisphere than in the southern. They change with difference in altitude on the slopes of mountains, as they do with differepce in latitude in travelling north or south, and are most easily perceived on the slopes of steep mountains, where the successive zonal bands may be compared without great. difficulty. Life-zones to the present time have been most inten sively studied in North America. By the student the life-zone is further divided into faunas, where conditions imposed by rain fall, geological formation and similar factors produce sections characterized by aggregations of species or sub-species.
Birds, in general, have attained a vast distribution over the surface of the globe because of their ability in flight and their specialization for life on both land and water. As a result of this versatility and adaptability some form of bird is found at some season everywhere over the world, except perhaps in the centre of the great unexplored Antarctic continent. Broad areas of the sea, away from the great ocean currents that flow like rivers through this aquatic medium, may appear birdless for days and weeks, but are crossed at certain seasons by some of the petrels in their wanderings. The seas and lands within the Arctic Circle are visited by many birds, some of which, as the raven and snow bunting, nest far north in Greenland, and some, as ducks and gulls, in summer traverse the solitudes of the north polar sea.
As individuals, birds may be tremendously abundant in tem perate regions, but it is within the Tropics that the greatest variety of forms occur within small limits. The largest aggregations are found in regions of diversified topography, where the life-zones change within a comparatively few miles from tropical to alpine.
The greatest number of forms at present recorded from a limited area is that reported by Dr. Frank M. Chapman from Ecuador, where within approximately 75,000 sq.m., an area less in extent than Great Britain, there are at present known 1,508 forms of birds, and it is said that this list is far from complete.