DEFINITION of a Bird. Strange as it may seem, the old proverb that “A bird is known by its feathers,” finds such exemplification in the science of today that it has actually become the scientific definition of the Class. It was formerly supposed that birds possessed a number of peculiar features, but, says Mr. Ridgway, “the most recent investigations of comparative anatomists have gradually eliminated the supposed exclusive characters of birds, as a class of the Animal Kingdom, until only the single one mentioned above, the possession of feathers, remains.” As Dr. Stejneger very aptly expresses it, ” No bird is without feathers, and no animal is invested with feathers except the bird.”
From the fact that in most systematic arrangements of the Animal Kingdom the Class embracing the birds is usually made to precede the mammals, it might be supposed that the birds were more or less closely related to them. As a matter of fact this is not so, for, according to Beddard, beyond the circumstance of “warmbloodedness and resemblance of some of the more simple forms of feathers to hairs, there is nothing to be said in behalf of a kinship between birds and mammals.” In some ways birds are undoubtedly the highest of the vertebrate animals. Their body temperature, higher than that of any other animals, is one index of their intense activity; their skeleton is perhaps more modified from the general type than that of mammals, their “arrangements for locomotion, breathing, and nutrition are certainly not less perfect,” and it is, in fact, “only when we emphasize the development of the nervous system and the closeness of connection between mother and offspring, that the mammals are seen to have a right to their preeminence over birds.”
It has long been settled with definiteness that the Class of birds is very closely allied to the Class of reptiles; for even at the present time, that is among living birds and reptiles, there are numerous structural points in common. It is, of course, perfectly easy to distinguish between a bird and a reptile as each exists at present, the most obvious difference being the warm blood and outer covering of feathers in the bird, and the cold blood and usually scaly covering among reptiles. On account of unfortunate breaks or gaps in the paleontological record, we are, with the exception of the evidence afforded by Archaeopteryx, in ignorance of the precise steps in the transition from one Class to the other, and may always remain so, but as was long ago pointed out by Huxley, Cope, Marsh, and others, it is probably in the group of extinct dinosaurs that we must look for evidences of this affinity. This large reptilian group embraces a number of forms, particularly those of carnivorous habits, that without doubt regularly progressed in a bipedal manner, and it was from the obvious similarity between these and the birds that Huxley concluded that “there could be no doubt that the hind quarters of the Dinosauria wonderfully approached that of birds in their general structure, therefore, that these extinct reptiles were more closely allied to birds than any which have lived.” Other anatomists have held that these resemblances in structure are simply adaptive, and came to be evolved in the dinosaurs from community of habit with birds, but very recently Professor H. F. Osborn has reexamined the evidence and has detected a number of additional points of agreement in skeletal characters, and concludes that this hypothesis is not to be discarded, but is to be “very seriously considered” in connection with the origin of birds. Still more recently (1905) Professor E. Ray Lankester, director of the British Museum [Natural History], says: “The reptiles which come nearest to them in structure are the Dinosaurs, especially these Dinosaurs (like Iguanodon) which walked on their hind legs and had only three toes to the foot.”
In any event it seems safe to assume that if not more intimately related, the dinosaurs and birds must at least have had a common ancestor. Within the past few months, Mr. W. P. Pycraft, the eminent avian anatomist of the British Museum, has presented an interesting speculation regarding the probable appearance of the ancestral or incipient birds anterior to Arch Archaeopteryx. He says: “From what we know of other types of vertebrates we may safely assume that these ancestral birds were of small size, and were probably also arboreal. And from the unmistakable signs of the shortening of the body in modern birds, the trunk was also relatively longer, as it certainly was in Archaeopteryx. From these two inferences we conclude, with some degree of probability, that these creatures, these ‘birds in the making,’ had substituted leaping for climbing about the trees. And from this there was but a short passage to leaping from tree to tree. In these movements we may reasonably suppose that the fore limbs were used for grasping at the end of the leap. The use of the fore limb for this work would naturally throw more work upon the inner digits I3 so that the work of selection would rapidly tend to the increased development of these, and the gradual decrease of the two outer and now useless members. Correlated with this trend in the evolution, the axillary membrane the skin between the inner border of the arm and the body became drawn out into a fold, while a similar fold came to extend from the shoulder to the wrist, as the fore limb, in adaptation to this new function, became more and more flexed. While the fingers, upon which safety now depended, were increasing in length, and growing more and more efficient, they were, at the same time, losing the power of lateral extension, and becoming more and more flexed upon the fore arm. And the growth in this direction was probably accompanied by the development of connective tissue and membrane along the hinder, post-axial border of the whole limb, tending to increase the breadth of the limb when extended preparatory to parachuting through space from one tree to another, the long claws being used to effect a hold at the end of the leap. The hind limbs, though to a less extent, were also affected by the leaping motion, resulting in the reduction of the toes to four, and the lengthening, and approximation of the metatarsals 24 to form a ‘cannon’ bone.
“The body clothing at this time was probably scale-like, the scales being of relatively large size and probably having a median ridge, or keel, recalling the keeled scales of many living reptiles. Those covering the incipient wing, growing longer, would still retain their overlapping arrangement, and hence those along the hinder border of the wing would, in their arrangement, simulate in appearance and function the quill feathers of their later descendants. As by selection their length increased, so also they probably became fimbriated, and more and more efficient in carrying the body through space.”
Temperature. It is a generally recognized fact that the temperature of birds is normally very high, but, strange enough, exact data on the subject are not extensive. Recently Mr. A. Southerland undertook to ascertain the temperature of certain ” Ratite ” birds, and one of the most interesting incidental facts brought out is the demonstration of a progressive increase from the lower to the higher birds; that is, the forms of birds that are regarded as the lowest in the scale exhibit the lowest normal temperature, while between these and the more active and highly organized there is almost every gradation. This condition also prevails at least to some extent among mammals. The Apteryx or Wingless Birds of New Zealand exhibit the lowest temperature thus far recorded among birds, the average of three individuals belonging to two species being 37.9°C. (100.2 ° F.). Next to the Apteryx come the Emeus, Cassowaries, and Penguins, with an average normal temperature of 39° C. (102.2° F.), while the Tinamous examined showed a range from 39.2°C. to 41.3° C., or an average of 4o.6° C. (105° F.), “which brings them up to the lower limit of the range of temperature usual for ducks, game birds,” etc. The common fowls when lifted quietly off their perches at night have a temperature of 40.6° C. (105° F.), but when lifted by day from nests whereon they are brooding, their temperature averages 41.7° C. (107° F.). From these birds there is another decided advance when we come to the great groups of small and excessively active birds such as Sparrows, Warblers, etc., their temperature ranging from 42° C. (107.6° F.) to 440 C. (111.2° F.), with an average of perhaps 109° F., or fully ten degrees above that of man.
Feathers. We may now advert to a consideration of the peculiar outer covering of birds; namely, the feathers. A normal feather (Fig. 1) consists of a hollow transparent basal portion called the barrel, or calamus, continuous with which is the main shaft, or rachis, which is opaque, roughly quadrangular in cross-section, and filled with a pithy substance. The rachis is furrowed along its inner surface; that is, on the side next the body of the bird. From the rachis above the barrel arise a “series of lateral branches, the barbs or rami, which in turn give rise to the barbules, and these to minute, often hooked processes, the barbicels”. It is by the hooking together of these processes that the web is produced and strength is given it to resist or act upon the air. Springing from the under side of the feather, in many cases, at the juncture of the barrel with the web-bearing portion, is a secondary feather, or aftershaft, as it is called.
In some birds, as in the Cassowaries and Emeus, the aftershaft is as large as, or larger than, the main portion of the feather, while in others it is greatly reduced or even absent.
Not all feathers exhibit as complicated a structure as that above described. In many cases the feather is much reduced, as for example in the wing-quill of the Cassowary, which consists merely of the stiff, naked stem or rachis, The so-called filo plume is another modification of the typical feather, a good example of which may be seen on the body of a common fowl after the removal of the outer or contour feathers It is to all appearances a slender hair, but in reality it is a degenerate feather which has a very short barrel and a thin, hair-like rachis with few or no branches. Usually the filoplumes are entirely concealed by the contour feathers, although in some birds, as the Cormorants, they form tufts of plumes on the sides and back of the neck which project beyond the outer (contour) feathers. A further modification is found in the so-called ” downs,” these being feathers in which there is no rachis, the long fluffy branches all arising at the top of the barrel: they are concealed by the contour feathers. In certain birds, such, for example, as the Herons, Bitterns, some Hawks, Par-rots, Tinamous, etc., the downs are aggregated in special patches, called ” powder-downs,” in which the ends are continually breaking off into fine, dust like particles. In the Herons, for instance, the powder-downs form enormous patches, a pair on the breast and a pair over the thighs. Their nature is not well understood, although their presence may constitute a well-marked character for descriptive purposes.
Colors of Feathers. The colors of feathers, or their apparent colors, constitute an exceedingly interesting phase of this subject. In accordance with the latest authorities the colors of feathers may be conveniently classed under three heads. The first of these, and the most general, are called chemical or absorption colors, since they are due to the presence of distinct coloring-matter. This coloring-matter may be in the form of a pigment, or may be a coloring solution, which is distributed in or among the cells composing the various parts of the feathers. Under this category come black, red, and brown, mostly the orange and yellow, but rarely green, and never blue. These colors may be recognized by the fact that they do not change under any condition of illumination or the position of the eye viewing them, and certain of them, as black, red, and yellow, may be separated in a practically pure state by well-known methods of chemical manipulation. One of the most interesting, known as turacin, is found in the red feathers of the Plantain Eaters (Musophagidae). It contains the same chemical elements as those found in black feathers, but with the addition of from 5 to 8 per cent of copper, and these birds lose their red color when washed by the rain, but regain it again when dry. The water in which they bathe is said to become reddish.
The second kind of coloring results from the combination of a pigment with certain structural peculiarities, such as ridges and furrows, in the surface of the feather itself. Such colors as violet and blue, usually green and sometimes yellow, belong under this heading. In transmitted light feathers with these colors show only the color of the pigment. ” For instance, the deep green or blue feathers of a Parrot will thus appear only gray or yellowish. The same happens when their polished surface is scratched or crushed; the blue color instantly disappears, showing only the blackish underlying pigment, or yellow pigment in green feathers. When thoroughly wetted in a bath the feathers of the back of an Amazon Parrot appear brown without a trace of green.” NEWTON.
The third form of coloring, or apparent coloring, includes the exquisitely beautiful prismatic or metallic colors, such as those found in Hummingbirds, the Birds of Paradise, Peacocks, Doves or Pigeons, Starlings, Grackles, and very many other birds. The manner in which these effects are produced has given rise to much speculation, and an extensive literature exists upon the subject, but even now the question can hardly be considered as definitely settled. The commonly accepted hypothesis is that metallic colors are due entirely to the structure of the surface of certain parts of the feathers, such as striae, ridges, knobs, or pits, in combination often with extremely thin, transparent, colorless layers, these elements, it is asserted, acting as prisms and changing the color as the direction of the light and the position of the eye change. Recently Dr. R. M. Strong appears to have demonstrated that the above explanation fails to meet certain of the physical requirements of the case. In investigating the metallic colors of feathers from the sides of the neck of the domestic Pigeon, he failed to find striae or other inequalities on the outer surface of the barbules that were sufficiently numerous or uniform enough to produce the observed colors by diffraction, but he did find that the barbules within the metallic area were strikingly different from those outside it. Instead of lying vertically, as in the non-metallic areas, they are turned so that one side faces upward, thus giving a much greater reflecting surface. The dorsal face of these barbules is provided with an outer transparent wall which encloses cell-cavities filled with pigment, this coloring-matter being in the form of spherical granules in the metallic-colored barbules, whereas in the non-metallic-colored areas the granules are of the typical rod shape. He therefore concludes that “the metallic colors of these feathers are probably thin-plate interference colors or Newton’s-rings effects, which are produced when spherical pigment granules come in contact with the outer transparent layer.” The whole subject of coloration is treated at length by Newton in his ” Dictionary of Birds,” to which the reader is referred for fuller information.
Pterylosis. It is a fact of common observation that in the hair covering of certain mammals, such as the horse, dog, cat, etc., the hairs are set as closely together as practically possible, forming a continuous covering. From the fact that in most birds, the entire body, except the beak and feet, is ordinarily covered by the feathers, it might be inferred that they are as continuously and evenly distributed over the body as are the hairs of mammals. This condition is far from true, however, as may be readily demonstrated by plucking the feathers from the body of any common bird, when it will be seen that the feathers are only borne on certain definite areas or tracts, being ordinarily spread out so as to more or less completely cover the body. This peculiarity was noticed and especially emphasized by Nitzsch, a celebrated German ornithologist, some sixty years ago.
The feather areas he called ” feather-forests,” or pterylae, and the naked spaces apteria, from the Greek, signifying “without feathers.” The description of the feather distribution in birds is called pterylosis or pterylography. The most important of the feather tracts are as follows : A spinal tract which runs down the backbone from the nape of the neck to the tail; a ventral tract, which “runs from the throat down the front of the neck, and dividing at its base, passes down on each side of the breast and abdomen to the inner side of the thighs quite to the end of the body”; the humeral tracts, a pair of tracts running across the upper arm, and forming what are called the scapulars; and the femoral tracts, a pair of tracts over the thighs. The pterylosis of the Nighthawk is shown in the accompanying figures (Figs. 3, 4).
It has been found that the extent and distribution of the feather tracts and bare spaces are relatively very uniform for certain groups of birds, so uniform, in fact, that pterylosis was made the basis of an elaborate scheme of classification of birds by Nitzsch. That it is of diagnostic value in many cases cannot be denied, but when relied upon too implicitly it not infrequently leads to what are obviously unnatural assemblages. Further than this we are still in ignorance of the pterylography of a vast number of birds, for the subject has been largely neglected since the time of Nitzsch. When a larger array of facts is at hand it may be possible to extend its usefulness.
The only birds at present known to have a continuous feather covering are the Penguins and Screamers, although formerly the Ostriches and their immediate allies were supposed to fall within this category. Apteria have long been known to occur in the embryos of certain “Ratites,” as the Ostrich, Rhea, and Apteryx, but recently Pycraft has shown that small but relatively important apteria occur in the adults of all members of this group.
Development of Pterylosis. In a communication before the American Ornithologists’ Union, November 18, 1903, Dr. Hubert Lyman Clark presented some important results of his examination into the development of the pterylosis. The material studied consisted of 54 embryos belonging mainly to different groups of birds such as Herons, Rails, Sparrows, Woodpeckers, Hummingbirds, and Swallows. In every instance he was able to distinguish the outlines of certain of the principal feather tracts before the body of the bird had assumed very definite form, for example, before the shape of the head could be distinctly made out. In all cases examined the caudal tract was the first to be outlined, and further the middle pair of tail-feathers was uniformly the first to appear. At this stage in the development of the embryo the tail was disproportionately long and had the feathers disposed along it in pairs, a condition very suggestive at least of the tail of Archaeopteryx. As a further interesting result it was shown that in the specimens studied the secondaries of the wing were the first to appear, thus confirming the result of Pycraft’s studies on the development of the Mound-Builders (Megapodidae). The primaries were found to be developed distally; that is, from the angle of the wing toward the tip. It would seem that characters as deep seated as these have been shown to be must have an important bearing on the taxonomy of birds when we are in possession of a sufficient body of facts to permit of generalization, but, as Dr. Clark pointed out, we do not yet know the complete history of the development of the pterylosis of a single species of bird.
The causes which have led to the development of feather tracts and consequent bare spaces are not well understood. The suggestion that it is simply another example of nature’s economy of material seems hardly an adequate one; the explanation advocated by Mr. F. A. Lucas, namely, that it is a case of adaptation, being decidedly more logical. On this point Mr. Lucas says: “The pterylosis of all birds is more or less adaptive, having some direct relation to their habits, and this adaptation is well shown in Hummingbirds. The bare tracts on the nape and along the throat allow the neck to readily lie against the middle of the back, or to bend downward over the points of the breast-bone, while the bare spaces under the wings and along the sides of the body permit the wings to be easily and closely applied to the body, the sides conforming almost exactly to the curve of the edge of the folded wing. The large bare space on the under side, found in nearly all birds save the water fowl, is merely to allow the warmth of the body to be directly applied to the eggs during incubation, and in birds like Ducks and Penguins (also Auks), which are densely or completely feathered beneath, a bare space is present during the breeding season.”
Renewal of Feather Covering. Although a considerable proportion of the feathers in flying birds are relatively strong, especially those of the wings and tail, the more or less active life of their owner results sooner or later in the wear and often injury of all feathers. The chief of the destructive influences to which feathers are subjected are abrasion and fading, the one a mechanical disintegration, and the other a chemical decoloration. Besides these there are other minor factors, such as “the age of a feather, its position, its structure, its color, and the habits of the bird.” It is thus evident that if a bird were provided with only a single set of feathers, they would ultimately become so worn and frayed as to be useless, either as a covering for the body or for flight. By a wise provision of nature, the entire feathering is renewed at periodic intervals. This renewal, known as the moult, takes place normally once a year, usually after the arduous duties of rearing the young are over ; but there are numerous exceptions to this, some birds acquiring two, three, or exceptionally even four, more or less complete annual changes of plumage, and further, these moult periods ” must not be confounded with occasional new growth at any time and anywhere to replace feathers accidentally torn out.” In order that the ordinary activities may not be seriously interfered with during the period of moult, there is a distinct relation between the feather loss and feather gain, most birds at no time being deprived of either the power of flight or the protection afforded the body by the feathers, and furthermore this fall and replacement is more or less synchronous from the opposite sides of the body. Thus in probably all of the great group of Passerine birds, the moult of the flight feathers begins in the middle of the wings with the practically simultaneous fall of the proximal or innermost primary on each side. As soon as the old feather has fallen, the new-forming feather pushes into view, and grows rapidly, and by the time the expanded portion of the feather itself is breaking from the apex of the follicle the next primary falls, and so the renewal by pairs proceeds outwards. With the secondaries the renewal proceeds in the opposite direction, that is the outer or distal one on each side falls first, this loss being very nearly coincident with the fall of the fifth or sixth primary, and their replacement by pairs proceeds towards the body. In some cases, as in Ducks, Geese, Swans, and Flamingos, the wing-quills are all shed at once, thus rendering the birds practically helpless for a short time, but this is very exceptional. The feathers of the tail are also normally renewed in pairs, the central pair falling first, followed by the quills next adjacent on either side. The process, however, is much more rapid than in the wings, for by the time the outer pair has fallen the middle ones are often not half grown. The renewal of the body feathering is in less obvious sequence, though “the moult regularly begins at fairly definite points in the feather tracts, radiating from them in such manner that the outer rows of feathers where the tracts are widest, and their extremities are normally the last to be replaced.” DWIGHT.
Renewal of Parts other than Feathers. Although the renewal of the plumage is the most important event of this kind, the feathers are by no means the only part of the integument that is periodically changed. Thus in certain Grouse the claws or pectinations along the sides of the toes become greatly lengthened in winter and are partially shed or worn down in spring and summer; the Puffins and Auklets shed portions at the base of the bill and around the angle of the mouth; the Penguins, or at least the King Penguin, moults the bright orange-colored membrane at the base of the bill; the American White Pelican develops a curious appendage on the upper mandible which is shed at the close of the nesting period. These phenomena and others of similar character will be more fully described under various forms exhibiting them.
Age of a Feather. After a feather attains its maturity the contents of the quill dry up and it is incapable of further development. It becomes, so to speak, “dead.” A great deal has been written to prove the contrary; namely, that a feather may, after it reaches its maturity and does duty for a varying length of time, again take on a period of active growth and change. This is to account for certain so-called “changes of plumage without moult.” But it seems to be now settled beyond any reasonable doubt that when a feather completes its growth, and the contents of the quill become dry and “lifeless,” it can never reinaugurate the process. There may be a fading or bleaching, or the outer extremities of the feather may be abraded in one manner or another, resulting oftentimes in a freshening or brightening of the plumage, as for example in the spring plumage of Purple Finches, Crossbills, etc., but this is quite different from a renewed growth and repigmentation of the individual feathers. This brightening of the plumage in the above mentioned species was long ago shown to result from the fact that the winter feathers have red barbs and gray barbules, and on the latter wearing away, the red elements of the feather are exposed. Beyond these relatively slight changes by abrasion or disintegration of the tips, or by fading, change of plumage can only result from moult.
It is only within the last few years that any extended researches have been undertaken in this country with a view of elucidating the sequence of plumages and moults in our birds, and while an extensive body of facts has been accumulated, much still remains unknown. Those who wish to look further into this subject should consult the papers mentioned below by Witmer Stone 1 and Dr. Jonathan Dwight, Jr.2 The latter author has placed the terminology of the subject on a uniform and logical basis. His scheme will be understood from the following tabular arrangement :
1. Natal (Down).
2. Juvenal (‘First plumage’).
3. First winter.
4. First nuptial.
5. Second or adult winter.
6. Second or adult nuptial.
3. First prenuptial.
4. First postnuptial.
5. Second or adult prenuptial.
6. Second or adult postnuptial.
Nests and Eggs of Birds. A word should perhaps be said in regard to the nests and eggs of birds, but in the space at command it can be but the briefest outline of a broad and extremely complicated field. It is perhaps unnecessary to state that all existing birds, without a known exception, lay eggs. To a casual observer, viewing a large collection of birds’ eggs, the variation in size, shape, color, markings, etc., seems almost infinite. For the differences in size he is prepared from his knowledge of the known differences in the sizes of birds producing them, yet there are noticeable surprises even in this respect. But the attempt to account for the striking differences in shape, color, and markings of eggs seems an almost hopeless one, yet such attempts have been made, though, it must be confessed, with varying degrees of success. Theoretically we may be justified in presuming that the lowest of existing birds, that is those that are supposed to approach most closely to their reptilian ancestors, should produce eggs or have habits of nidification showing the closest approach to the reptiles. An examination of the facts, however, shows that this relationship is capable of demonstration to a limited degree only. In the first place, it appears that much is still to be learned regarding the oölogy of existing reptiles. Broadly speaking, the eggs of all reptiles are white in color and spherical or ellipsoidal in shape. Nothing is at present known regarding the eggs of either the ancient reptilian or avian ancestors of our birds, although, as Shufeldt has suggested, it is by no means impossible that such remains may sometime be found, especially those of the Toothed Birds. Fossil eggs of turtles have been obtained, as well as fossil or subfossil eggs of a few species of birds, but they throw little or no light on the question at issue. It is, then, from this hypothetical starting-point that all the marvelous diversity observable in the eggs of living birds has been developed, but the steps by which it has been accomplished are, and must perhaps remain, obscure. Natural selection has doubtless played a most important rôle, though the “whys and wherefores” are far from being satisfactorily answered. There are certain salient groups of facts that stand out boldly, yet the exceptions are so numerous and marked as to prevent establishing any adequate chain of cause and effect. Thus practically all birds that nest in holes, such as the Woodpeckers, Kingfishers, Bee-eaters, Rollers, Hornbills, Barbets, Puff-birds, Trogons, Toucans, Parrots, Parakeets, and Swifts, lay white eggs, yet the coördinate groups of Owls, Hummingbirds, and Pigeons, that build an open nest, also lay white eggs, while many birds that habitually nest in holes lay spotted or even richly colored eggs. Again, it is often possible to trace a marked similarity in pattern of coloration throughout nearly all the species of a whole natural family, or a large genus, but for which “the conditions of environment offer no explanation, since it as often occurs in cosmopolitan groups as in those of local distribution, and which, in the present state of our knowledge, seems wholly inexplicable.”
Theory of Birds’ Nests. So, also, are we without an adequate theory of birds’ nests. “Why the thousands of species of birds,” says Dr. J. A. Allen, “build each a peculiar nest, differing more or less in situation and architecture from those of all other species, is a question which has yet received no satisfactory answer. As a rule the nest, including its location, the materials and manner of its constructure, is as distinctive of the species as the number, size, form, and color of the eggs, or, in some instances, as any fact in its history, not excepting even the details of structure and coloration of the bird itself. Why this is so we can perhaps explain when we can satisfactorily account for the diversity of song that is scarcely less a scientific characteristic.”
The attempt has been made to explain it on the ground of a connection between the colors of the female and the mode of nidification. That is, according to Wallace, “when both sexes are of strikingly gay and conspicuous colors, the nest is such as to conceal the sitting bird, while, whenever there is a striking contrast of colors, the male being gay and conspicuous and the female dull and obscure, the nest is open and the sitting bird exposed to view.” This condition undoubtedly prevails in many cases, but the exceptions are so numerous and so important, that the “theory” fails of adequacy.