| ARCHÆOPTERYX |
| Skeleton of Archæopteryx macrura with indication of feathers |
| (Reconstructed. After Andrea) |
The oldest fossil bird known is that named archæopteryx, whose remains are found in the Jurassic slates of Bavaria, which represent the beginning of the Mesozoic or Age of Reptiles. In much of its anatomy, and in the possession of perfect feathers, it is a true bird, yet it retains many reptilian features. Its body was about the size of a small crow; its legs were rather long, with well-developed feet of four toes suitable to grasping a perch; its wings were short and probably feeble, for the shoulder girdle and ribs are weak and the sternum is rudimentary. It is plain that it was arboreal in habits, but a poor flyer, and was aided in scrambling about the branches of trees on whose leaves and bark it may have fed, by the fact that three digits of the rather lizardlike wing hand terminated in strong claws, while the thumb was entirely free.
The practical value of this clawed hand is illustrated in a living bird—the hoatzin, of northern South America—which exhibits in several ways the probable appearance and manners of the archæopteryx. "It haunts the sides of lagoons and rivers where a thick growth of low trees projects over the stream or the mud left bare by the tide. When disturbed the bird flies off awkwardly with a violent flapping motion, or leaps from bough to bough, erecting its crest and expanding its wings and tail. The note is sharp and shrill, and has been described as a hissing screech. The food consists of leaves and fruit. The conspicuous nest, placed on low trees or shrubs, is a loose platform of spiny sticks and twigs with a softer lining, and contains from three to five yellowish eggs, spotted with reddish brown and lilac. The young, which can see and run as soon as they are hatched, have a claw on both forefinger and thumb, by means of which they creep about the thickets, and hook themselves over the branches, assisted by the bill and feet. They can also swim and dive."
The most striking features of the archæopteryx were its head and tail. The skull is fairly avine, and the rather short and blunt bill was furnished with conical teeth, nearly equal in size, and set in a marginal row in distinct sockets. Still more lizardlike was the tail—a prolongation of the backbone nearly as long as the body, along each side of which sprouted strong feathers forming a horizontally flat tail with a rounded end.
| TOOTHED BIRD |
| (Hesperornis regalis) |
| Skeleton of toothed bird (After Marsh) |
The next that we know of bird evolution is derived from the discovery of the fossil remains of toothed birds in the Upper Cretaceous formations of Kansas—that is, in the more recent half of the Mesozoic Age. They differ greatly not only from archæopteryx but from each other, and are represented by several species. One type (Hesperornis) was a wingless, diving bird of great size, whose long, heronlike beak was studded with small, sharp teeth, all alike, implanted in a continuous groove; its legs were so hinged to the compressed pelvis that they could be extended almost level with the back, and the lobed toes thus became lateral winglike paddles of great power. The other type, represented by Ichthyornis and its relatives, also had a long, stout bill set with teeth, but each in a separate socket. Ichthyornis was about the size of a pigeon, and its strongly developed wing bones and deeply keeled sternum show that it was a bird of powerful flight, and apparently gull-like habits. So far as we know neither of these Cretaceous birds had any progeny. When, after an immensely long period, other fossils come to light in rocks of the middle Tertiary period they bear few traces of ancestry, and exhibit little relation to the great mass of modern orders. They are the "flightless birds," possessing no wings but running about on massive legs; and the group includes the extinct æpyornis, dinornis, and moa, and the existing ostriches, rheas, emus, cassowaries, and kiwis. Some ornithologists question whether this "ratite" group, characterized by having no "keel" on the sternum, did not have an origin and line of descent quite distinct from those of both the Cretaceous toothed birds and the modern "carinate" type which possess a medial crest or "keel" on the breastbone for the support of the flight muscles; but the more general opinion is that they are a variant from very early birds with wings.
HOW A BIRD IS BUILT
Since its feathers are the one thing that marks a bird, outwardly, as different from other classes of animals, we ought first of all to learn what feathers are, and what purpose they serve. A quill feather, such as may be picked up in any farmyard, has a horny, hollow stem or "shaft," which is bare at the closed large end or "base," but has two soft, winglike expansions toward its tapering end that together make its "vane." This thin, flat vane consists of delicate branches, "barbs," studded with tiny hooks, the "barbules," holding each adjacent branchlet in place, but letting the whole vane bend and spring. The whole beautiful thing is really very strong and elastic, as it must be to push as hard against the air as a bird's wing has to do. The vanes vary much in shape, and in the degree to which the branchlets are disconnected into a fluffy looseness. Ostrich plumes, and those of the birds of paradise, owe their beauty to the fact that each branch in the vane is loose, and bears little disconnected branches of its own; and in many feathers no vane at all grows, so that they resemble hairs, when fine, and bristles when coarse, as is seen about the mouth of the whippoorwill and some flycatchers. The nestling plumage or "down" is of this character. The lovely plumes of egrets are slender stems of feathers having in place of a vane scattered soft hairs. In some sea birds the feathers are so stiff and hard as to be almost like scales. Those of water birds, and especially the divers, are wonderfully close, thick, and greasy, so that the down that forms an undercoat for warmth, and the skin beneath it, never gets wet.
Feathers, then, serve their wearers first of all as clothing—very thick and warm in birds of cold places; and doubtless this beneficial modification of the primitive reptilian scale, by reason of its conserving the warmth of the body, and gradually increasing the temperature of the blood, has been largely instrumental in enabling birds to rise so far above the grade of their cold-blooded and sluggish ancestors.