The Encyclopedia Americana (1920)/Birds

BIRDS. The birds form that class (Aves) of warm-blooded vertebrate animals most distinctive, most easily defined and most popularly known and interesting. They are at once distinguished by their covering of feathers, which is possessed by no other sort of animal; and by the modification of their fore-limbs into instruments for flight (wings). Their aerial existence, from which few have wholly departed, requiring great activity and exertion, has called forth a high perfection of organization, especially in the respiratory and circulatory systems of the body, and has led to the characteristic spindle-shaped form, narrowing from the full chest and shoulders toward a pointed head, which will cleave the air easily, and diminishing toward the rudder-like tail. The graceful form, to which the beauty of birds is largely due, has been brought about by the enlargement of the shoulder-girdle, and its great pectoral muscles, and by the necessity of increased capacity of chest to contain the comparatively great heart and lungs. In birds such as ostriches, cassowaries, moas and the like, which have ceased to fly, and have developed very strong legs; or in those like the penguins, which have become swimmers and divers, the changes of structure are degenerations from the type, which is a bird with powers of flight.

Character of the Feather Coat.&mdash; Flight, as well as clothing, is due to the presence of the complicated horny appendages growing from the skin, called feathers, which are peculiar to the class. Their structure is described under. Those of the body are usually small, grow in certain definite tracts

(see ), varying in the different groups, and form a close jacket, not easily pervious to moisture and a poor conductor of heat, thus conserving the vital warmth and protecting the body against sudden changes of temperature. It is shed (molted) and renewed semi-annually. This body-coat is ordinarily nearly uniform in length and character, but often is varied by ornamental plumes, erectile crests, ruffs, and other modifications, such as are seen in birds of paradise, herons and many others. The feathers are also variously colored in patterns varying with the groups and more minutely with the species, whereby they may recognize each other and be distinguished by us. These colors are usually those of pigments incorporated in the web of the feather itself, but may be due to minute scales on the surface, which break up the light, giving it an iridescent or metallic sheen, conspicuous in hummingbirds and in certain pheasants. The plumage often varies, according to age, sex, season, or all three conditions; and these colors play an important part in bird-life. See .

Powers of Flight.&mdash; The bones of the wing and tail support very large, strong &ldquo;quill&rdquo; feathers, which, when outspread, support the bird in the air, and when moved in the proper manner carry it forward &mdash; enable it to fly, the mechanism and phenomena of which method of locomotion are explained under. The wing-power of most birds is very great, but the speed of their flight is often exaggerated. Few exact facts are at hand, but it is apparent that the highest speed is nearer 50 than 100 miles an hour, although the latter speed is undoubtedly reached by ducks and other swift flyers under pressure of attack or escape. Endurance on the wing is more remarkable. Many seabirds seem tireless, and swallows, among land-birds, are almost incessantly in the air. During migrations a large variety of birds, including some of the smallest and feeblest, undertake rapid and extensive journeys, reaching in some cases almost half around the world; and some regularly pass over spaces of ocean as much as 2,000 miles in width, while a flight of 500 miles from land to land is accomplished by many species. This is the more notable as a feat because in many cases they are birds which during nine-tenths of the year only flit from bush to bush. In these migratory journeys (see ) birds often fly very high; but this is the regular custom of certain ones, especially vultures, which soar beyond human sight, yet will swoop to the earth in a swift dash, betraying great adaptability to sudden changes in atmospheric density. Other notable qualities are the power (largely residing in the tail) to suddenly change speed and direction, helping them to dodge and elude winged pursuers, and to catch the agile aerial insects, upon which many of the smaller species depend for subsistence. The sharpness and quick adjustability of eyesight also involved in this is noteworthy.

These abilities in flight have led to the very wide distribution of birds, which occur in every part of the world yet seen by man; and are the most numerously represented of all terrestrial branches of animal life in the oceanic islands. Nevertheless very few are cosmopolitan, and

not many range beyond the confines of a single continent, while many are more narrowly restricted, so far at least as their habitat in the breeding season is concerned. Thus the geographical distribution of birds has been found perhaps the best criterion for the mapping out of zoogeographical regions (see ). The greater number of families of birds is tropical, and both variety of kinds and numbers of individuals decrease toward the poles. A striking fact is the great difference between the birds of the northern and the southern hemispheres &mdash; a difference much more decided than exists between those of Europe and North America, or of South America and Australasia.

Reproduction.&mdash; Birds in every case reproduce their kind by means of eggs protected by a calcareous, often highly-colored shell, laid by the mother a considerable time before they are ready to hatch, which consummation is brought about by the application of warmth. This may be arranged for in two ways. A few birds bury their eggs in rotting vegetation, or in hot sand, and let the chemical heat evolved by the ferment in the former case, or the sun's rays in the latter, accomplish the desired result. The great majority, however, place their eggs in some sort of a receptacle (sometimes a mere hollow on the ground, or hole or niche in a cliff or tree, sometimes in a burrow or nest of more or less elaborate construction ; ( see ), and there brood upon, or &ldquo;incubate&rdquo; them until the chick matures and emerges. In one class of birds (Præcoces) incubation is so long continued, and the embryonic chick becomes so far advanced before leaving the shell, that it is well coated with feathers, and can at once begin to lake care of itself. These birds are the sea-birds, water-birds, game-birds and their allies of comparatively low organization. In another class (Altrices) of higher organization as a group, the chicks are permitted to break from the shell before they have acquired feathers or are able to move about or obtain food. They must therefore be shielded, defended, fed and cared for by the parents for several days or weeks. Out of this condition have grown some of the most interesting, complicated and delightful features, habits and instincts of bird-life.

Food and Feeding Methods.&mdash; Birds as a class are omnivorous, but each of the various groups might be characterized by its food, which, more than anything else in the process of evolution, has determined the various types of structure that distinguish their tribes, and which are indexed, as it were, by the form of the bill and feet. Those of lowest organization, &mdash; nearest the ancestral type, &mdash; are the sea-birds, which live upon fish varied to some extent by mussels and other small marine creatures. Many of the ducks and shore-birds share this marine diet, and numerous wading birds eat fresh-water fish, frogs, crayfish and the like. The great body of ratite and gallinaceous birds, &mdash; ostriches, emeus, partridges, pheasants, etc., that run and nest on the ground, &mdash; are vegetable-eaters, seeking green leaves, fruits, seeds, lichens, etc., and picking up such insects as come in their way. All the foregoing are præcocial birds, and the young feed on the same things as their parents. These classes have little relation to mankind so far as their food

is concerned except that they sometimes devour too much grain or spoil certain plants. Among the higher class, or altricial birds, the fare is more varied, and while there is a very numerous group (the cone-billed or fringilline birds; see, etc.), which live altogether upon seeds, and a few others, like the kingfishers, which catch fish, the great majority indulge themselves in a miscellaneous diet of both vegetable and animal materials. Some called &ldquo;soft-billed,&rdquo; and including most of our song-birds, except the finches, are mainly insect eaters, some catching them upon the wing, others digging them out of rotten wood, and the greater number picking them off the leaves of trees or searching for them among the herbage. Another large class, embracing the birds of prey, and a few others, like the shrikes, depend for food upon capturing and devouring other smaller birds, together with such small mammals, reptiles, amphibians, fish and insects as they are able to seize and kill. These are the falcons, owls and their relatives; but a related group, the vultures, varies this fare by feeding upon carrion.

Usefulness to Man.&mdash; In the case of all of these altricial birds, however, except the birds of prey, the young are fed upon soft insect food, mainly worms, caterpillars and maggots; and the period of their nesting coincides with the time when these larval insects abound. In the feeding habits of these higher birds man has a great interest, for nearly all of the innumerable insects which they capture for themselves, or for the nourishment of their young, are such as are annoying or injurious to him; and experience in many localities has shown that the destruction of bird-life is accompanied by a distressing increase of noxious insects. In the same way the hawks and owls, by their incessant pursuit of mice, and other small animals injurious to agriculture, so reduce the numbers of these pests, as greatly to benefit the farmer; while the useful work done by the vultures, as scavengers, by removing offal and dead animals, is recognized by everyone in the tropical regions where these birds most abound.

Nor does the relative usefulness of birds to man stop here. They not only afford him great pleasure, by their pleasing colors and animated behavior, and delight his ear by their voices, but large numbers of them furnish him with excellent and even dainty food. Lastly, this group has furnished men with several varieties of domestic poultry, such as the turkey, peacock, guinea-fowl, duck, goose and various pigeons and partridges, that are among the most valuable of his animal possessions.

Distinctive Character of North American Bird-Life.&mdash; In considering the bird-life of North America, the natural southern boundary is the arid region between the United States and Mexico, which acts as an effective barrier. Our bird-fauna is by this means substantially separated from that of Central and South America. On the contrary it strongly resembles that of northern Europe and Asia. While a large number of extensive tropical families are not known north of central Mexico, or are represented by only one or two species, many of the families, the genera and even a fair number of species &mdash; not counting the seabirds common to both oceanic coasts &mdash; of the United

States and Canada are the same as those of Europe, and, to a less degree, of Siberia. This is especially true of the ducks, geese, shore-birds and birds of prey. When we examine the list of birds of the interior fields and woodlands the general resemblance to those of Europe remains close, while their divergence from those of South America is very striking. In fact, Europe has only two families of small birds not also represented in North America &mdash; the starlings and the wrynecks.

The explanation of this condition, like that of so many other facts in our natural history, is to be looked for in the past. Geologists find evidence that in the early part of the Tertiary period, and at intervals during its progress, the northern borders of this continent were connected with both Europe and Asia; and that at times in the past the subarctic climate was comparatively warm &mdash; perhaps like that of the Middle States now.

This state of things enabled birds and many other animals of the Old World (which there is reason to believe was provided with animal life before this continent) to pass hither to the New; and abundant time has elapsed since for them to spread all over the continent, and to undergo the changes promoted by the differences in climate and food, which have resulted in the promotion of new American species, all derived from a remote, Old-World ancestry. This history gives a reasonable explanation of both the likeness and the diversity between the two faunas.

Distribution of Birds in North America.&mdash; One of the most important features in ornithology is the study of geographical distribution (q.v.). Few species of birds extend their range over a whole continent, even in their seasonal migrations; almost all are restricted to a comparatively small area, the limits of which are set principally by their requirements in each case in the matter of food suitable not only for themselves but for their young in the nest.

The birds of widest range are those whose food is widely distributed, such as swallows, seed-eating sparrows, river-haunting sandpipers and ducks, and the predatory hawks and owls that feed on small animals. Even these are more or less affected by local food-conditions.

But the food available to and suitable for birds depends very largely on climate; and climates vary according to geographical conditions. Hence, climate is the greatest factor in determining the range of any species of birds. But by climate in this connection we mean the climate of summer, that is the average degree of warmth and moisture &mdash; the average kind of weather during the season when a bird is hatching and rearing its young.

On this continent the isotherms, or lines of equal warmth, do not run straight across from east to west according to the latitude, because of the interference made by our two great north-and-south mountain systems, whose crests are colder than are the low countries between them. Therefore in the middle west they bend northward and run up toward Alaska. Thus a species that is comfortable in Connecticut, but finds Maine too cold, will reside in Michigan, Minnesota and halfway to Alaska &mdash; perhaps 20 degrees north of the latitude of Maine &mdash; because it finds there an

mid-summer warmth like that of southern New England. Thus are constituted approximately east-and-west districts, or &ldquo;zones,&rdquo; of life inhabited and characterized by groups of birds requiring similar conditions.

Thus characteristic Arctic birds do not come much south of the treeless region extending in the west from Great Bear Lake to a point half-way down the coasts of Hudson Bay and into northern Labrador. These dwell in the &ldquo;Arctic&rdquo; Zone. South of it lies the narrow &ldquo;Hudsonian&rdquo; Zone, which swings from the mouth of the Saint Lawrence along the southern shore of Hudson Bay and northwestward to Alaska, including all of that peninsula. South of this the &ldquo;Canadian&rdquo; Zone embraces all the wooded parts of Canada except the Saint Lawrence Valley, and sends a long tongue down the Mackenzie River almost to the Arctic Circle. South of this lies the &ldquo;Transition&rdquo; Zone &mdash; a narrow strip running from New England and New York through southern Quebec and Ontario west and northwest to the prairies of the Dakotas and western Canada; it also extends far south along the cool heights of the Alleghanies and the Rockies. The remainder of the United States east of the Rockies is included in the &ldquo;Austral&rdquo; Zone, which is divided into an &ldquo;Upper&rdquo; and a &ldquo;Lower&rdquo; half, the latter embracing the South Atlantic and Gulf States. The Rocky Mountains form a composite zoological district where altitude has much the same effect on local climate, and consequently on animal life, as has latitude elsewhere. West of them the Pacific Coast is a &ldquo;region&rdquo; itself, with many exclusive birds.

Each of these zones has a bird-population which is not to be found outside of it in the breeding season; and that is true also of the vegetation and of other kinds of animals, showing the interdependence of all forms of life, and the reason for the facts noted in the distribution of North American birds.

Migration of American Birds.&mdash; The general subject of migration (q.v.) is treated elsewhere; but a few words may be pertinently added in this connection as to local peculiarities. Here, as elsewhere, migratory habits are adapted to local conditions especially as to routes followed; and here as elsewhere most of these are far from keeping to the precise north-and-south direction that many persons suppose birds always take.

In the first place many of our birds make no regular migration at all. These are known as &ldquo;residents.&rdquo; They are such as are able to find food all the year round in the region where they live; but in the case of some of these there is a partial migration, the individuals of a species moving a short distance southward from the northern border of its range into a more favorable climate, chiefly to escape deep snow. Then a considerable number of species of birds of northern Canada are forced southward in the fall, and visit the region of the Great Lakes, northern New York and New England during the cold weather, but rarely go farther south. These are the true &ldquo;winter birds.&rdquo;

Running over the list one will find that of about 1,000 species of North American birds only about 225 depart in winter as far south

as the Gulf States, and of these only two-thirds entirely leave the United States. The fact that these far-travelers include almost all of the song-birds, or those most noticeable in summer, makes it seem to the uninformed northerner as if the whole bird-tribe had left the country.

Federal Protection of American Birds.&mdash; The Federal migratory bird regulations of the United States prohibit throughout the United States the killing at any time of the following birds: Band-tailed pigeon; little brown sandhill and whooping cranes; wood duck, swans; curlews, willet, upland plover and all shore birds (except the black bellied and golden plovers, Wilson snipe or jacksnipe, woodcock, and the greater and lesser yellowlegs); bobolinks, catbirds, chickadees, cuckoos, flickers, fly-catchers, grosbeaks, hummingbirds, kinglets, martins, meadowlarks, nighthawks (or bull bats), nuthatches, orioles, robins, shrikes, swallows, swifts, tanagers, titmice, thrushes, vireos, warblers, waxwings, whippoorwills, woodpeckers and wrens, and all other perching birds that feed entirely or chiefly on insects.

Books about North American Birds.&mdash; Under will be found a list of books to be consulted by the scientific student. It is desirable here to mention a few books of general interest and giving an account of the bird-life of special districts. For birds of the world generally: Evans, &lsquo;Birds&rsquo; (New York 1900) Beebe, &lsquo;The Bird&rsquo; (ib. 1906); Knowlton and Ridgway, &lsquo;Birds of the World&rsquo; (ib. 1909). For the Eastern States; Chapman, &lsquo;Handbook of Birds of Eastern North America&rsquo; (New York 1912); Chapman, &lsquo;Bird-life&rsquo; (New York 1901); Stearns and Coues, &lsquo;New England Bird-life&rsquo; (Boston 1904}; Eaton, &lsquo;Birds of New York&rsquo; (2 vols., col. plates, Albany 1910-12); and books by Thoreau, Torrey, Hoffman, Wright, Abbot, Knight, Parkhurst, Merriam, Miller, Ekstorm, Sharpe and other popular writers. For the Mississippi Valley Wheaton, &lsquo;Birds of Ohio&rsquo; (Columbus 1891); Dawson, &lsquo;Birds of Ohio&rsquo; (Columbus 1903); Butler, &lsquo;Birds of Indiana&rsquo; (Indianapolis 1890); Barrows, &lsquo;Michigan Bird-life&rsquo; (Lansing 1912); Cory, &lsquo;Birds of Illinois and Wisconsin&rsquo; (Chicago 1910); Goss, &lsquo;Birds of Kansas&rsquo; (Topeka 1899); Coues, &lsquo;Birds of the Northwest&rsquo; (Washington 1874). For the western United States, besides many government documents: Coues, &lsquo;Birds of the Colorado Valley&rsquo; (Washington 1878); Cooper, &lsquo;Ornithology of California&rsquo; (Sacramento 1872); Bailey, &lsquo;Handbook of Birds of the Western United States&rsquo; (Boston 1912); Keyser, &lsquo;Birds of the Rockies&rsquo; (Chicago 1902); Wheelock, &lsquo;Birds of California&rsquo; (San Francisco 1904); and writings of Dawson, Grinnell, Bryant, Henshaw, Keeler, Merriam, etc. For the Southern States: Bailey, &lsquo;Birds of Virginia&rsquo; (Richmond 1912); Wayne, &lsquo;Birds of South Carolina&rsquo; (Charleston 1910). For Canada: Dionne, &lsquo;Oiseaux de la Province de Quebec&rsquo; (Quebec 1906); McIlwraith, &lsquo;Birds of Ontario&rsquo; (Toronto 1894); Macoun, &lsquo;Catalogue of Birds of Canada&rsquo; (Ottawa 1909); and several valuable documents of the Canadian and United States government relating to western Canada. The files of the ornithological magazines, The Auk (New

York), Bird-Lore (New York), Wilson Bulletin (Ohio), and The Condor (California), should also be consulted. Also the &lsquo;Bulletins&rsquo; and &lsquo;North American Fauna&rsquo; (series) of the United States Biological Survey; the &lsquo;Bulletins&rsquo; of the United States Department of Agriculture; and the &lsquo;Proceedings&rsquo; of the United States National Museum.