TYPES OF LIVING BIRDS Mammals have specialised for life on land (as the deer and mouse), in water (as the whale), in the air (as the bat), for night life (as the lemur), and in the earth (as the mole). But birds specialise for existence or progression through the air. Though it is common for birds to nest in holes or tunnels in the earth, which, in many cases, are excavated specially for this purpose, no bird has been able to adapt itself to continuous life under ground, probably because its covering of feathers is not de signed to withstand the constant abrasion that would take place in burrowing. Also, though many birds frequent the water, none are so wholly adapted to life beneath the surface as the whales. The bird thus has failed to utilize completely the ecological environments that have been available for it—a mani festation of its inferior adaptability compared to the mammal.
Birds adapted to a terrestrial existence are found in many widely separated groups, as for example ostriches, bustards, plovers, larks and pipits. Most of these adopt a terrestrial habit to secure food and safety from enemies. They run about on the earth, and though the greater number retain power for flight, often trust to their legs to avoid or escape ordinary pursuit. Most of them show greatest variety in regions of extensive plains, prairies or broad, open downs. The wholly terrestrial forms that have entirely lost the power of flight are comparatively few, and (so far as Continental areas are concerned) are birds of considerable size and strength. Among living forms they include the ostriches of Africa, which extend into Eurasia as far as southern Persia, the cassowaries of New Guinea and adjacent areas, the emus of Australia, the kiwis of New Zealand and the rheas of South America. These birds, though united by certain peculiarities, differ widely from one another, and seem to represent ancient types of bird that were formerly more abundant, as a number of fossil forms are known. It will be noted that large flightless birds exist in modern times in all of the continents except North America, where they have not been represented since the Eocene.
The kiwis (Apteryx) of New Zealand, the queerest and most unbirdlike of living birds, are nocturnal and have different habits from the other species mentioned. They have long bills with the nostrils at the extreme tip, and move about by using the bill to test the ground before them as a blind man uses a cane, noting their surroundings partly by touch and partly by smell. On casual
examination they seem to have no wings, but on investigation the wing is found concealed beneath the feathers, a tiny structure, a little more than 3 in. long when fully extended, and entirely without flying function, as the bird is heavier than an ordinary fowl. The cassowaries, like the kiwis, inhabit forest areas, but are far less peculiar. The wings are small, with heavy, naked quills nearly concealed beneath the long, hair-like feathers, and there is a curious casque on the head. It is said that the bird uses both casque and wing-quills to fend off entangling vines in travelling through the jungle.
Ostriches and rheas have larger wings and may extend them while running, but are wholly without power of flight. In all these birds the body is heavy and the legs are large and strong. With those fleetest in running there is a tendency to reduction in the number of toes, so that the rhea has three and the ostrich only two. That these birds have come from flying ancestors is apparently indicated by the wing, which, while small and weak, has the bones formed as in flying species. All the species dis cussed have the external surface of the breast bone smooth without the great keel so characteristic of the flying birds, there being no necessity for such attachment, as in flightless birds the muscles usually concerned in flying have little development.
There is another type of terrestrial bird that has developed more recently than those just discussed, found mainly on oceanic islands. This includes various species of flightless rails, and some other birds that have lived under conditions in which they had no enemies from whom it has been necessary to escape by flying, so that through disuse the power of flight has been lessened until, finally, the birds are not able to rise from the ground, though the wing retains the same form encountered in flying birds and the breast bone still has a keel, though this may be greatly reduced in size. The weka rails of New Zealand are the most striking living examples of this group; they are nearly as large as a fowl, and run with great ease and rapidity. Some other flightless rails are much smaller, for example the Laysan rail (Porzanula Palmeri), of Laysan island in the Hawaiian islands, which is not much larger than a newly-hatched chicken. In running fast this bird extends its wings and flaps them rapidly, but is unable to rise from the ground. Many other insular birds show a tendency to degeneration of the wing but can still fly.