THE distribution of life over the globe is known as biogeography. With that branch of the subject dealing with the distribution of animals (zoögeography) we are of course at present only concerned with the distribution of birds. Although earlier attempts had been made to correlate certain observed facts of avian distribution, it was not until about the middle of the last century that the subject was placed on a logical and scientific basis. These earlier attempts failed because it was undertaken to delimit life areas by degrees of latitude and longitude, or by the lines of political division, while in the light of present understanding it needs but a moment’s reflection to disclose the fact that the distribution of life on the earth must depend upon natural causes and conditions, and only occasionally and quite by accident to coincide with the political divisions.
In as much as most birds possess the power of flight, which enables them to pass easily and quickly from one area to another, or, within certain limitations, even from one hemisphere to another, it might be inferred that their distribution would be relatively uniform, but such is far from being the case. If a person reasonably familiar with the bird life of eastern North America should journey around the world, first crossing this continent, and thence by way of Central America through South America, and by way of Polynesia to New Zealand, Australia, Asia, Europe, and Africa, he could not fail to be impressed with the marked differences in the birds coming under notice. In a few cases, such as afforded by the pelagic and far-wandering Albatrosses, Petrels, Gulls, Terns, and Tropic-birds, he might find the same species at several widely separated points in the journey; in many instances he would note the presence of familiar groups, as Ducks, Woodpeckers, Kingfishers, Hawks, Owls, but in the vast majority of cases the species would be entirely different, while in not a few, whole families and orders would appear and disappear in succession.
It is not always easy to account for the presence or absence of certain genera or other groups of birds in this or that part of the world, especially when it appears that the conditions of environment in localities whence they are absent are apparently similar to those obtaining where they are present, and were it not for the aid rendered by geology and paleontology we should often be left without an adequate explanation. It is no doubt true that certain groups of birds now confined to circumscribed areas could exist as well in other parts of the world where climatic and food conditions are practically similar, provided the avenues for reaching them were open. For instance, the abundant fresh-water bodies of the New World, teeming with fishes and other aquatic life, seem admirably adapted to support a varied Kingfisher fauna, and doubtless our Troupials (Icteridae) could change places with the Old World Starlings (Sturnidae), or our Wood-warblers (Mniotiltidae) with the Old World Warblers (Sylviidae). These groups, and of course many others could be mentioned, appear to have reached their present standing in approximately the same geographical areas they now occupy, and long since all land connection between their respective habitats has been cut off.
In the light of geological evidence it is beyond dispute that in the past numerous large land masses have been many times joined together and many times rent asunder. In this way continents as well as lesser land areas have been successively joined and cut off from one another, and each junction has left the way open for an exchange of life forms. North and South America have been many times severed and united; Australia by a relatively slight subsidence has lost New Zealand, and Madagascar was undoubtedly at one time a part of the continent of Africa. Both New Zealand and Madagascar were stocked with certain animal forms while they retained connection with their parent masses which the sea barrier has since prevented from commingling, at least to any great extent. The important bearing of these facts on distribution is obvious.
Another and very important geological event was the glacial epoch. This vast ice-mass, sweeping down for hundreds of miles from the pole, profoundly modified the life, not only in the area actually covered by the ice, but far beyond its actual border. Many forms must have been crushed out of existence, while others, enjoying perhaps better means of migration, were pushed before it towards the tropics, which resulted in intensifying the struggle for existence in an area probably already well stocked. The forms of life from both sources that could not readily adapt themselves to the changed conditions were pushed to the wall and left little or no trace of their existence. By the recession of the ice, territory was gradually reclaimed, which was occupied by the surviving forms, with the exception of those typically northern forms remaining permanently stranded in southern mountain areas.
To paleontology we are also indebted for some contributory data regarding distribution, for while the fossil remains of birds are not very numerous, they are often sufficiently so to show that many groups once enjoyed a much wider distribution than now. Paleontology thus makes plainer the possible lines of travel by which the descendants of the ancient forms have reached their present locations. But this class of facts is far less important than those last considered, for the paleontological record is less complete for birds than for almost any other group. It is mainly of value in fixing the antiquity and affinities of certain groups, and even here it is often distressingly meager.
Of the causes controlling or influencing distribution it is generally admitted that temperature and humidity are the chief factors, and, according to Merriam,”it has been found in the case of mammals and birds that the effects of tempera- estimated numerically, are more than three times greater than the effects of humidity upon genera, and many times greater upon the higher groups.” While there is some difference of opinion as to the exact period during which temperature exerts the greatest influence, “there can be but little doubt that for both animals and plants it is the season of reproductive activity.”
There are various other factors, aside from those already mentioned, that are known to exert a greater or less influence on geographical distribution. The character of the soil, which carries with it an effect on the plant and insect life, may be mentioned, as well as the mechanical purity of the atmosphere as evidenced by the prevalence of fogs, etc. Deforestation, the usual mark of the advent of civilization, has quite markedly affected distribution, and the extension of cultivated areas by means of irrigation over lands previously arid has increased the habitable areas for some species, and has also resulted in displacing many indigenous forms. Mountain ranges have often been considered to be efficient barriers against distribution, and that they have an effect is true, but it is mainly the effect of altitude and temperature ; for if conditions are similar on opposite sides of a range, they will usually be found inhabited by the same forms, which may have reached these positions by passing around the extremities of the mountains or by means of passes through them. The real barrier is climate and not mass. Thus both sides of the Rocky Mountains as well as the high Sierra in California, are found to be inhabited by the same species of birds, and, says Merriam: ” The great Himalaya has little or no influence in bringing about the really enormous differences that exist between the faunas and floras of the plains on its two sides, for these dissimilarities are due primarily to the great difference of temperature resulting from unequal base level, the Thibetan plateau on the north being several thousand feet higher than the plain on the south.”
Oceanic bodies of water have of course a powerful effect on distribution, especially of land birds, but even here certain limitations must be borne in mind. To purely terrestrial animals the presence of even a moderate width of open water may prove an efficient barrier, but to birds, endowed as they are with the power of flight, it is less so than might be supposed. The Galapagos Islands lying six hundred miles off the coast of Peru have been stocked with an abundant fauna of land birds, evidently of South American and West Indian origin, and the Azores, seven hundred miles distant from South Europe, have a fauna of one hundred and twenty species and subspecies of birds, all Old World forms, the ranks of which are being yearly augmented by fresh arrivals. Within the last fifty years the White-eye (Zosterops caerulescens), a small passerine bird about the size of our Parula Warbler, has crossed over the twelve hundred miles of open water separating Australia from New Zealand, and has extensively and permanently colonized the latter; the European Widgeon and Ruff have again and again been found in the middle and western United States, and the American Catbird has been taken in Italy. Examples of this erratic wandering, or apparently regular journeying, might be continued almost indefinitely, but enough has been given to show that the sea is not an insuperable barrier in all cases.
It has been found possible to divide the land-masses of the world into a number of faunal (and floral) areas, each of which is more or less strongly characterized by the presence or marked absence of certain dominant or peculiar forms of life. The failure of early attempts at such delimitation was due, as already pointed out, to the effort to make them conform to the lines of political division, or to degrees of latitude and longitude ; and while naturalists are even now not in accord as to the number of primary divisions that should be recognized, Mr. P. L. Sclater was the first to put the subject on a scientific basis by applying to it a logical principle. The contention that “convenience, intelligibility, and custom should largely guide us” in prescribing life areas has long been discredited, for it is now obvious that the mere size of an area can have no real weight so long as it is sufficiently characterized. It should not be inferred, however, that these life areas, whatever their size and grade, are always sharply circumscribed by hard and fast lines; for while it is possible to define them with considerable definiteness in a general way, it rarely happens that a change from one to another is abrupt. Perhaps the most notable example of a sharp line of demarcation is that passing between the islands of Bali and Lombok and separating the Australian and Indian Regions. There is usually an area of greater or less width in which there is a commingling of the life forms of adjacent divisions, a neutral ground or transition area, as it is called. On the whole, however, it is found that these lines correspond quite closely to isothermal lines, or the lines of equal temperature.
Sclater demonstrated that the surface of the globe exhibited six great divisions, each of which differed in a marked manner from all the rest, though the difference was not always equally important. These divisions, which he called Regions, are as follows: Palaearctic, Ethiopian, Indian, Australian, Nearctic, and Neotropical. For upwards of twenty-five years most writers on the subject accepted this classification with little change, though the conviction slowly gained ground that the distinction between the fauna of the northern portions of the Old and New Worlds was not as pronounced as had been thought. In 1893 Dr. J. A. Allen proposed a new classification, denominating the divisions of the first rank Realms, while those of second rank were called Regions, those of the third rank Provinces, of the fourth rank Subprovinces or Districts, and those of fifth rank Faunas. He recognized seven Realms: Arctic, North Temperate, American Tropical, Indo-African, South American Temperate, Australian, and Lemurian. In the same year Professor Alfred Newton proposed an arrangement which retained the number of Regions recognized by Sclater, but their outlines were very different. They are as follows: New Zealand, Australian, Neotropical, Holarctic, Ethiopian, and Indian, and while in some respects it seems somewhat less logical than Dr. Allen’s, it is more conveniently followed here.
The New Zealand Region. There has been considerable discussion as to the propriety of considering this as a life area of primary rank, but notwithstanding the fact that it is by far the smaller of the areas usually so considered, it is sufficiently well characterized to warrant this distinction, and in fact cannot well be referred to any other Region. Regarding the matter of size it may be pointed out that while it is now restricted, if it is considered with relation to its dependent islands Norfolk, Lord Howe’s, and Kermadec Islands on the north, Chatham Islands on the east, and Auckland, Macquarie, and Antipodes groups on the south which were once undoubtedly a part of it, it is seen that the area is but little short of that of Australia. This New Zealand area was set off from Australia by subsidence at a remote period, geologically speaking, and has apparently remained continuously separated. As might be supposed, this early separation and continuous isolation has resulted in developing or perpetuating some very remarkable life forms. It is, or rather was until recently, the home of two perfectly distinct orders of birds, the Dinornithiformes, or Moas, and their allies, embracing two families, some seven genera, and about thirty nominal species, and the A pterygiformes, or Kiwis, of which there are six species.
The Australian Region. This Region, says Newton, “has but little connection with New Zealand and is as trenchantly divided from the Indian, which geographically, and possibly geologically, seems to be conterminous with it, by the narrow but deep channel that separates the small islands of Bali and Lombok, and will be found to determine the boundary between these two distinct Regions.” Starting with an imaginary line between these two islands, we may trace it north-easterly, passing between Borneo and Celebes, and between the Philippines and the Pelew group. Thence the line proceeds northward to the vicinity of the Tropic of Cancer, and then eastward somewhat indefinitely so as to include the Hawaiian Islands, though these are perhaps more North American, but to include all of which is commonly called Polynesia, and return so as to encompass the New Caledonian Islands and of course Australia proper as well as Tasmania.
Without going as extensively into the subject as is really warranted, it may be stated that the Australian Region is the exclusive home of the order Casuariiformes, comprising the Cassowaries and Emeus, the superfamily Pseudoscines, which embraces the families Menuridae (Lyre-birds) and Atrichornithidae (Scrub-birds), and the families Rhinochetidae (Kagu), Didunculidae (Tooth-billed Pigeons), Loriidae (Lories), Paradiseidae (Paradise-birds), and Ptilonorhynchidae (Bower-birds). In addition to these the following families are almost peculiar : Meliphagidae (Honey-suckers), Campephagidae (Cuckoo Shrikes), Artamidae (Wood-Swallows), Cacatuidae (Cockatoos), and Megapodidae (Mound-builders). As it is oftentimes nearly or quite as important to note the groups that are wanting in an area as it is to determine those which are present, it may be noted that the Australian Region lacks the families Vulturidae (Vultures), Phasianidae (Pheasants), and Pycnonotidae (Bulbuls), to which should perhaps be added the Fringillidae (True Finches), though the exact relationship of the so-called Australian Finches can hardly be considered as settled. The Picidae (Woodpeckers) are practically absent, as hardly half a dozen species cross the line into Lombok, Celebes, or the Moluccas, but do not occur elsewhere.
The Neotropical Region. The present Region comprises the whole continent of South America as well as Central America and the West Indies, extending as far north as the southern line of the Holarctic Region, which will be defined later. Although there are no life forms in common between the two last-mentioned Regions and the present, there is a certain alliance, as was long ago suggested by Professor Huxley, though of quite a different nature from any thus far considered. ” South America, that is to say the most important part of the Neotropical Region, retains a greater proportion of the less modified descendants of generalized ornithic types than does any other portion of the globe, the two Regions before mentioned only excepted.” NEWTON. In other words, this area appears to have been more or less completely isolated for a very long period of time, and as a result a large proportion of the archaic elements have been retained, albeit in modified form. Among the birds, then, we find a large number of peculiar types, including the orders Rheiformes (Rheas) and Crypturiformes (Tinamous), and some twenty-seven families or subfamilies, as follows: Procniatidae (Swallow Tanagers), Ccerebidae (Honey-Creepers), Zeledoniidae (Wren-Thrushes), Catamblyrhynchidae (Plush-capped Finches), Ptilogonatidae (Silky Flycatchers), Dulidae (Palm Chats), Oxyruncidae (Sharp-bills), Pipridae (Manakins), Cotingidae (Cotingas), Phytotomidae (Plant-cutters), Dendrocolaptidae (Wood-Hewers), Formicariidae (Ant-birds), Pteroptochidae (Tapacolas), Rhamphastidae (Toucans), Bucconidae (Puff-birds), Galbulidae (Jacamars), Todidae (Todies), Momotidae (Motmots), Steatornithidae (Oil-birds), Cracidae (Curassows), Opisthocomidae (Hoactzin), Chionidae (Sheath-bills), Thinocoridae (Seed-snipe), Cariamidae (Cariamas), Aramidae (Courlans), Psophiidae (Trumpeters), Eurypygidae (Sun-bitterns), Palamedeidae (Screamers).
In addition to the above-mentioned peculiar forms the following families are mainly Neotropical, only relatively few species crossing the line into the Holarctic Region: Icteridae (Troupials), Tanagridae (Tanagers), Tyrannidae (Tyrant-birds), and Trochilidae (Hummingbirds).
The Holarctic Region. Notwithstanding the fact that this vast Region is, area for area, much larger than all the others combined, it is the most difficult to satisfactorily define. Even the outlines are still open to question in many places, and can be given only approximately. In the New World the southern line of the Holarctic Region has been worked out with considerable detail, being in fact the northern line of the tropics. Starting on the Pacific coast, it crosses the lower portion of the peninsula of Lower California, thence crossing to the Mexican mainland it skirts the mountains and passes over into the Atlantic drainage in southern Mexico, when it bends northward to leave the continent in extreme southern Texas just above the mouth of the Rio Grande. Crossing the Gulf of Mexico, it excludes a portion of southern Florida and the Bermudas and touches the African coast about the vicinity of Mogador, where its course is clear, for it follows the northern limit of the Great Desert. As regards its extension across the Asiatic continent to the Pacific we may accept the recent statement of Mr. H. E. Dresser, who supposes it to “run to the northward of the Arabian Desert, and including the tableland of Persia, the highlands of Baluchistan, the whole of Afghanistan, and the Himalayan Range above about ó000 feet, stretching to the south of Tibet, and north of the valley of Yang-tse-kiang as far as the Pacific, and then around Corea, and the main islands of Japan.”
The Holarctic Region may be divided into two major areas (denominated Regions by Sclater), called the Nearctic for that portion embraced in the New World, and Palaearctic for the Old World portion. In the entire Nearctic area there is but a single peculiar family of birds, the Chamaeidae, or Wren-tits. All the other Nearctic families are common to the Neotropical Region or to the Palaearctic area or to both. This condition of affairs also prevails in the Palaearctic area, for of the twelve or thirteen hundred species of birds inhabiting it, there is not a single family that is absolutely peculiar to it. For the proper characterization of this Region we must consider the genera, but the presentation of these data in sufficient detail to become intelligible would take us quite beyond the limits of the space available.
The Ethiopian Region. This comprises all of the continent of Africa below the northern border of the Great Desert, together with much of Arabia and the extreme southern portion of Persia. There is some doubt as to the propriety of including the great island of Madagascar, as some naturalists notably Dr. Allen would accord it primary rank, and there can be no doubt that it possesses some remarkable life forms. The closest affinity of the Ethiopian Region appears to be with the Indian Region which touches it on the east, and Dr. Allen has combined the two areas as the Indo-African Region. Among the many interesting birds at least the following groups are peculiar: Orders Struthioniformes (Ostriches) and the extinct AEpyornithiformes (Elephant-birds), the suborders Gypogerani (Secretary-bird), and Mesaenatidae (Mesite), the superfamily Scopidae (Umbrette), and the families or subfamilies Balaenicipitidae (Shoe-bill), Musophagidae (Plantain-eaters), Leptosomatidae (Kirumbos), Coliidae (Colies), Philepittidae (Asitys), Hyposittidae (Coral-billed Nuthatch), Vangidae (Vanga Shrikes), AErocharidae (Helmet-bird).
The Indian or Oriental Region. This, the last of the six Regions to be defined, is bordered by the Ethiopian region on the west, the indefinitely outlined Holarctic Region on the north, and the rather sharply circumscribed Australian Region on the east. While this Region is the home of a large number of peculiar genera and species, there appears to be but a single peculiar family; namely, the Eurylaemidae, or Broad-bills.