The above should, however, be somewhat expanded, in order to shew in greater detail the differences between Birds and other Vertebrata. Care must nevertheless be taken to avoid the fault common to many modern definitions, of giving an abstract of the main characteristics of the object, rather than a clear guide to distinction.
Dr. Gadow[[1]] defines Birds as "oviparous, warm-blooded, amniotic Vertebrates, which have their anterior extremities transformed into wings. Metacarpus and fingers carrying feathers or quills. With an intertarsal joint. Not more than four toes, of which the first is the hallux."
Much of this the beginner might well postpone, his attention being solely drawn to the external characters; though of course those that are internal are by no means to be subsequently neglected. Indeed no satisfactory progress can be made in the serious study of Ornithology, or the Science of Birds, without a competent knowledge of their Anatomy and Development; while, though at present comparatively few fossil remains of Birds have been found, some of them are of the highest importance, and there is every probability of future discoveries throwing much light not only on the mutual relationships of Birds among themselves, but also on their connexion with the Reptilia. Birds are, in fact, only extremely modified Reptiles, the two Classes forming the Sauropsida of Huxley, one of his three primary divisions of Vertebrata. The aid of the Palaeontologist and Geologist must thus be called in to clear up many problems which present themselves to the Ornithologist who does not content himself with examining existing forms of life alone. Archaeopteryx (p. [23]) from the Jurassic System is the oldest Bird known, nor are any other pre-Tertiary forms recorded, save a small number from the rocks of the Cretaceous Epoch, the chief of which are the so-called Odontornithes, or toothed species of America (p. [45]).
The following paragraphs on the structure of Birds will help to explain the systematic account in the later chapters.
Feathers.–Returning to the outward character denoted by the popular saying with which we began, the Feathers[[2]] constituting the plumage may not inconveniently be first considered. The general belief that they grow from almost every part of a Bird's body, as do hairs in most Mammals, is erroneous; for, almost without exception, they grow in certain definite tracts called pterylae, the intervening spaces, whether they be wholly bare or covered with down, being termed apteria. The arrangement of these patches is at times of considerable assistance in determining a Bird's affinities; and the subject may be studied in Nitzsch's Pterylographie[[3]] or in a shorter form in Dr. Gadow's article "Pterylosis" in Professor Newton's Dictionary of Birds.
A feather originates thus. A conical papilla arises in the derma and pushes up the epidermis, a depression forming meanwhile around the base; subsequently the derma supplies a nutritive pulp, while part of the epidermal layer is converted into a tuft of stiff rays, meeting and forming a short tube below; these thereafter burst their covering and protrude as the rami or barbs, on which, apparently by secondary splitting, are commonly produced radii or barbules. In this state we have a plumule or "down-feather"; but in the case of the feathers that have "webs" or "vanes" (vexilla) often called contour feathers (pennae or plumae), a fresh papilla forms at a deeper level, so that the earlier structure is thrust forward and eventually drops off from the apex of the later. Meanwhile the "dorsal" portions of the barrel or quill (calamus or scapus) at the base of the tuft of rays have elongated into a principal shaft (rhachis); this is generally accompanied by a secondary "aftershaft" (hyporhachis), originating from the "ventral" side, which in the Emeu and Cassowary rivals the shaft itself in size. On the rhachis a double series of lamellae or barbs are developed, carrying a similar double series of barbules, much as in the down-feather, but the barbules again give rise to barbicels (cilia), which in the distal rows usually terminate in hooklets (hamuli). These catch in the folded margins of the next proximal row, and a firm surface is thus secured. An after-shaft never, and a down-feather rarely, possesses barbicels; while in some cases by the absence of these and part of the barbules a "disconnected" web and a "decomposed" feather are formed, as in the decorative tufts of many species. The barbs may even be absent, as in the wing-quills of Cassowaries, the wires of Birds-of-Paradise, the "bristle-feathers" at the gape of Night-jars or the eyelashes of Hornbills. In the hackles of Gallus (Fowl), and the secondaries or even the tail-feathers of Ampelis (Waxwing), the tip of the rhachis is flattened and wax-like; and similar structures are observable elsewhere. In the newly-hatched young the down is often partly or entirely suppressed, but in certain Birds this suppression is temporary, and a thick coat grows after a few days. "Powder-down" feathers are those which never develop beyond the early stage, and continually disintegrate at the tip into bluish- or greyish-white powder; they occur in the Tinamidae, Ardeidae, Rhinochetidae, Eurypygidae, Mesitidae, Accipitres and Psittaci, in Podargus, Coracias, Leptosoma, Gymnoderus and Artamus.
Colour.–The colour of Feathers is due to one of three causes. First, an actual pigment[[4]] may be present in certain corpuscles, or in diffused solution, and the tint does not then vary according to the incidence of the light. Secondly, it may arise from a pigment overlaid by colourless structures in the form of ridges or imbedded polygonal bodies; here, if the vanes are scraped or held up to the light, the pigmentary colour alone is visible.[[5]] Thirdly, the colour may be iridescent or prismatic; that is, a blackish pigment may lie beneath a surface, which, whether polished, ridged, or pitted, acts as a series of prisms, causing the hue to vary according to the relative position of the spectator's eye and the light. This is seen in a remarkable degree in Humming-birds.[[6]]
Not uncommonly the vanes of feathers have an appearance like watered silk, due to very indistinct transverse striations. In regard to plumage generally, it may be noticed that the markings on a feather frequently indicate the age of a bird. In some the immature plumage is characterised by light-coloured tips to the feathers, which are lost as maturity is reached. In other groups, and especially in most of the Accipitres or Diurnal Birds of Prey, the markings of the immature bird are generally longitudinal, and in the adult transverse. In nearly all these cases the change is effected at the first moult. Females and young are usually duller than males, but in some cases, such as Phalaropus (Limicolae) and Eclectus (Psittaci), the hen-birds are the more brightly coloured.
Moult.–Referring to p. [2], it should be remarked that, after the production of a feather, the formative substances become for a while dormant, but awake to renewed activity, if accidental or periodical loss needs to be made good; and so we naturally arrive at the phenomena of the annual Moult, which is often double, that is, occurring towards autumn, and again in spring.
Though some Birds do not lose their quill-feathers the first year, they normally gain a winter plumage–differing in colour from the summer garb–by moulting or shedding their feathers. The wing-quills, and even those of the tail, are ordinarily discarded in pairs, though not quite simultaneously; but most Anatidae (Swans, Geese and Ducks), and apparently the Phoenicopteridae (Flamingos), lose all the former at once,[[7]] and with them the power of flight; while in the first-named Family the males of many species assume for several weeks a dress resembling that of the female, and are said to undergo an "eclipse." Young birds moult, as a rule, somewhat later than adults, but in the typical Gallinae the original quills are shed before the possessors are fully grown, and are succeeded by others of proportionately increased size, the power of flight being attained very early.