10. The organs of voice in Birds have long attracted special interest from the loud cries which some utter, and the melody with which others are gifted. Setting aside the part played by the trachea or windpipe in supplying air to the lungs, its formation is worthy of attention. Its upper end consists of the larynx, and it passes down the neck as a flexible tube, formed by a continuous succession of bony rings connected by membrane, until it bifurcates into two bronchi, which open into the lungs. A common feature, found in many groups not nearly allied, is the dilatation of a portion, generally near the middle, while a remarkable modification is exhibited by the males of many of the Duck-tribe, some of the lowest rings being fused together and forming what is known as the bulla ossea or "labyrinth." In other Anatidae (some of the Swans), and some of the Cranes, the trachea enters the keel of the sternum; but a not unfrequent modification, usually confined to the male sex, often occurs elsewhere, when the windpipe is looped back upon itself. All these arrangements, however they may affect the sounds uttered by Birds, do not in themselves constitute the voice organ of most. That is reserved for the syrinx, a peculiarity of the Class Aves, consisting of the lower end of the trachea and the adjoining part of the bronchial tubes; and the varied modulations are effected by means of muscles attached thereto. These voice-muscles may be wholly absent or of the simplest character, but they attain their highest perfection in the Passeres, and especially in the large group of them known as Oscines, where there are often five or seven pairs. In this group the lowest four or five tracheal rings are solidly fused into a little bony box communicating with the bronchi; the first and second bronchial rings (or in this part often semi-rings) being closely attached to the trachea, and the spaces between the second and third and the third and fourth being generally closed by an outer tympaniform (drum-like) membrane, while the rest of the semi-rings of the bronchi are closed by the inner tympaniform membrane. It should be clearly understood that all the notes emitted by Birds are produced by the above structures only, and that the tongue has nothing to do with their utterance, except, possibly, in the case of the sounds that Parrots (but not other birds) are taught to produce.
Classification.–The Classification of Birds is still in a condition of uncertainty, notwithstanding the many schemes successively propounded during more than two centuries. To dwell upon them here would be impossible, and it is only practicable to trace in the briefest way the line which has led to the most recent attempts, and to name those whose researches have produced the results which may be fairly regarded as attained. First among them is Nitzsch (1806-1840), to whom followed Merrem (1812-1817), and after a few years L'Herminier (1827). These three worked quite independently, and in their lifetime little notice was taken of their labours; for, though there were good ornithologists among their contemporaries, little value was then set upon internal characters in this connexion. An improvement took place when the great Johannes Müller (1846, 1847) published his scheme for grouping the Passeres, which, though based on purely anatomical facts, was almost immediately accepted, chiefly through the simultaneous exertions of Dr. Cabanis, by systematists of the Old School. For twenty years no advance was made, for the morphological researches of Parker were not directly taxonomical; but Huxley (1867, 1868) started what was practically a new line of investigation, though it subsequently appeared that up to a certain point it had been already suggested by Dr. Cornay (1842-1847). The impetus thus given was fortunately sustained, Huxley's example being followed by Dr. Murie, and by two promising men, A. Garrod and W. A. Forbes, both of whom died at an early age, leaving their mark in work which, though much of it was crude, was that of true genius. Mr. Sclater (1880) has tried to bring the results of the whole four into harmony with pre-existing views, and a similar attempt was that of Dr. Stejneger (1885); but all were overshadowed by the monumental performance of Prof. Fürbringer, whose Untersuchungen zur Morphologie und Systematik der Vögel, completed in 1888, must ever remain a record of unexampled labour, while his considerations on the derivation of Birds from Reptiles, and of the later groups of Birds from the earlier, whether his results be right or wrong, are of the utmost importance to the ornithologist. During the progress of this work the author was in frequent communication with Dr. Gadow, himself engaged on the ornithological portion of Bronn's Thier-Reich, and thus the opinions of each were in many cases mutually affected. Dr. Gadow, on the completion of his undertaking, propounded a scheme of classification, which is followed, with some slight modifications, in the present volume (see foregoing table)–it being, of course, understood that a linear arrangement is, strictly speaking, impossible, since any group may have a decided affinity to more than two others. This Classification, beginning (as Birds themselves must have begun) with the lower forms, takes us, except in the Oscines, as far as the Families, which in most cases are fairly distinguishable, though of very variable value. Coming to Genera, and still more to Species, the opinions of authorities often differ so widely, that at present an attempt to reconcile them is hopeless. It cannot be denied that Genera and Species are merely "convenient bundles," and that divisions of either, if carried too far, defeat the object for which Classification is intended. Genera are only more distinct from Species, and Species from Races, because the intervening links have disappeared; and, if we could have before us the complete series which, according to the doctrine of Evolution, has at some time existed, neither Genus nor Species would be capable of definition, any more than are Races in many cases; while the same remark will apply to the larger groups.
From these Races or Geographical variations we may not unnaturally turn to Geographical Distribution. It will always be credited to Ornithology that the interesting study of the Geographical Distribution of Animals was first placed on a scientific basis as a result of the study of Birds. This was effected by Mr. Sclater, whose division of the Globe into Six "Regions"–the Palaearctic, Ethiopian, Indian, and Australian, forming one group–the "Old World" (Palaeogaea); and the Nearctic and Neotropical, forming a second–the "New World" (Neogaea); was announced in 1858 (J. Linn. Soc. ii. pp. 130-145). His scheme, being solely grounded on Ornithological considerations, was accepted with scarcely any modification by Mr. Wallace in his great work (Geograph. Distrib. of Animals, 1876), and by the majority of zoologists, though some demurred, and among them Huxley, who, in especial reference to Birds, shewed (Proc. Zool. Soc. 1868, pp. 313-319) that there was more reason to divide the earth's surface latitudinally than longitudinally, and that Four Regions were better than Six–these four being (1) Arctogaea, comprising Mr. Sclater's Indian, Ethiopian, Palaearctic, and Nearctic; (2) Austro-Columbia, corresponding with the Neotropical; (3) Australasia; and (4) New Zealand–the last three being combined as Notogaea. In 1882 Prof. Heilprin proposed to unite Mr. Sclater's Palaearctic and Nearctic under the name of Triarctic; but in the next year (Nature, xxvii. p. 606) adopted for that union Prof. Newton's earlier term Holarctic. Some other general schemes have been promulgated, as those of M. Trouessart and Professor Möbius; but they have found little support, and with regard to the Class Aves, though certainly not with regard to other groups as Pisces, or Mollusca, what is practically the scheme of Mr. Sclater has met with acceptance, whether with or without the modifications proposed by Huxley and Professor Newton, there being really but two important points of difference–(1) the recognition of New Zealand as a distinct Region, and (2) the union of the Nearctic and Palaearctic areas into a single Region. It would be impossible here to set forth the arguments by which these views are maintained or contested, and it must suffice to trace briefly the outlines of the several districts. New Zealand, if admitted as a distinct Region, consists only of the islands so named, the smaller Chatham, Auckland, and Macquarie groups, Antipodes Island, Lord Howe's, Norfolk and Kermadec Islands. The Australian, if the preceding be cut off, will include Tasmania, all Australia, and the islands to the northward as far as what has been called "Wallace's Line" (between Lombok and Bali), Celebes, New Guinea, New Britain, and all the countless groups of tropical islands in the Pacific Ocean–except the Galapagos, which undoubtedly belong to the next Region. The Neotropical is made up of all South America, the Antilles and Central America, the only doubt being whether to draw the northern boundary so as to exclude or include Mexico, or even the southern part of the United States. To this naturally succeeds, but with an indefinite southern boundary, the Nearctic, comprising the whole of the rest of North America to the shores of the Polar Sea, with the addition of Greenland. Its north-western corner, Alaska, is now known to be largely tenanted by forms from Asia, not found elsewhere in America, and this is one of the chief reasons assigned for uniting it with the Palaearctic area, which may be taken to include Japan and all continental Asia to the north of China proper, the Himalayas, the Persian Gulf and the east end of the Mediterranean. Some authorities would add Northern Arabia and Lower Egypt; but all have agreed to include Tunis and the ancient Mauritania–the Barbary States lying north of the Great Desert to the Atlantic Ocean about Mogador, as well as the Canaries, Madeira and the Azores, with the whole of Europe from Greece to Iceland. What is left of Arabia and Africa, after taking off the above portions, with the addition of Madagascar and the Mascarene Islands, is the Ethiopian Region; and all the rest of continental Asia, with the islands not included in the Australian Region, becomes the Indian, or, as it has lately been called, the Oriental. It would be quite impossible to enumerate here the various Sub-regions and Provinces into which these several Regions may be divided. The views of Mr. Wallace are set forth at length in his excellent work, those of Mr. Sclater in The Ibis for 1891, pp. 514-557, and those of Professor Newton in his Dictionary of Birds. Many writers would assign to Madagascar a higher rank than that of a Sub-region.
Migration.–Few peculiarities of Birds have excited more general interest than their seasonal Migration, which in many species is so marked as to have been observed from very remote times; and it is probable that nearly all species are subject to periodical movements of varying extent. These movements are greatest in the Birds which have their breeding quarters in the northern parts of the Northern Hemisphere; and, with some exceptions, it may be said that the more northerly is the range of a species the more extensive are its migratory wanderings. In the Southern Hemisphere the facts known are as yet insufficient to allow of safe deductions. Absence of a food-supply in winter is alone enough to account for migration in the above cases, and the return from the south in spring is probably due to the desire of Birds to reoccupy their old haunts, or those in which they have been bred. But just as there are some species which habitually breed within the Arctic Circle and winter in the Tropics, there are others which may not go so far in either direction, and yet have their movements governed by exactly the same principle, with the result that in a temperate zone we have Birds coming from the north to winter with us, while others, arriving from the south in spring, spend the summer here, and depart towards autumn. Others again, the true "Birds of Passage," arriving like the last in spring, make little or no stay, but pass onward to more northerly lands, and re-appear for as short a time in autumn on their return journey southwards. Moreover, observation shews that, in most parts of the temperate zone, there are many Birds which, though resident as species, are migratory as individuals–that is to say, that while examples of the species may be met with at certain spots throughout the whole year, those which occur at one season are not always the same individuals as those which occur at another–the particular Thrush, Titmouse, or Finch, appearing in the winter not being identical with that which appears in summer. Again, among species of which some individuals are constantly present throughout the year, a great accession to the numbers is made at the close of the breeding-season by the influx of other individuals of the same species bred in another district, though this influx generally lasts for a comparatively short time, and the strangers pass on, accompanied it may be, by some or even most of those that have been reared on the spot in the season immediately preceding. These species are the "Partial Migrants."
It would at first seem from the above that the annual migratory movement would be in a direction due north and south, or south and north, according to season, and so in a general way it is; but there is no doubt that this simple movement is disturbed by many causes, chief among which is possibly the configuration of the land, which is found to give rise to considerable deviations, and that to an extent which is at present very imperfectly understood. It may be considered proved that the trend of a coast-line, the course of a great river, or the intervention of a chain of mountains, has a very appreciable effect on the direction taken by migrating Birds; but not one of these, nor all in combination, affords a sufficient explanation of all the deflexions, and will certainly not account for at least one remarkable fact, as it may now be regarded–the tendency of many Birds in Eastern Europe and part of Siberia to travel westward towards the close of summer or in autumn. This is shewn in several ways, but in none better than by the almost yearly occurrence in Britain at that season of examples of species which breed only in the Russian Empire. For, admitting that such examples are stray wanderers, which have lost their course, their appearance here is still useful in indicating the existence of the westward movement; and, with the evidence they furnish before us, we may judge whence come vast numbers of others–Starlings, Crows, Rooks, Jays, Larks, and what not, whose origin and starting-point it would be otherwise hard to trace or even surmise. Much has been written, especially in Europe, on so-called Lines of Flight, but as yet to little purpose, and indeed scarcely any writers on the subject have had sufficient data to form an hypothesis, so that it is not surprising that hardly any two agree in theory.[[10]] In other parts of the world there is still less ground for theorising, though in North America many valuable observations have been made; and these, in conjunction with those carried on in Europe, will no doubt in due time lead to satisfactory results as regards the Northern Hemisphere. Concerning the Southern our ignorance is almost complete.
Of the way in which Migration is performed there is still much to learn–but one thing is certain, all Birds do not migrate in the same manner. Some gather in flocks, great or small, others seem to accomplish their northward journey in pairs, or at any-rate arrive at their breeding-quarters already paired. Some undoubtedly voyage by night, others may be seen to travel by day. Of the Birds which in spring arrive unpaired, it is now incontestable that the males outstrip or precede the females. There is, moreover, equal diversity in the southward movements towards the close of summer and all through the autumn. Of some species the earlier broods disappear without attracting attention, and the later broods as well as the parents slip away almost as imperceptibly. In one remarkable case, that of the Cuckoo, the adults leave this country long before the young are fit to follow; but, in by far the greater number, the young start first, and are followed, often at an interval of some weeks, by their parents.[[11]] It is contended by many that of actual Migration we see very little, since it is constantly carried on at a height where the Birds are beyond our ordinary observation, and as regards some species this seems to be true. Moreover, it would seem that the longest flights are performed by night, and when the sky is clear, so that only in thick weather do the Birds come near enough to the earth to be heard–seeing them being of course impossible in the dark, though in a few cases they have been telescopically observed passing across the face of the moon. It is certain that many of the smaller land-birds gradually press onwards prior to leaving our shores, but after that they may possibly betake themselves aloft to continue their journey.
The speed at which Birds travel during Migration is a matter on which very diverse opinions have been and are held; but the highest estimates, such as those of the late Herr Gätke (who would allow even 150 miles an hour), can scarcely be otherwise than exaggerations; for there is no evidence of any but exceptional performances at such rates, and there is really no reason to suppose that Birds can fly faster at a higher elevation than at a lower.
Fig. 6.–A Falcon. To shew the nomenclature of the external parts.
Terminology.–The annexed figure explains the nomenclature of most of the outward parts of a Bird, but some further explanations may be given, as below:–