Food and Feeding 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 organiza tion,— nearest the ancestral type,— are the sea birds, which live upon fish varied to some ex tent by mussels and other small marine crea tures. 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, — ostriches, emeus, partridges, pheasants, etc., that run and nest on the ground,— are vege table-eaters, seeking green leaves, fruits, seeds, lichens, etc., and picldng up such insects as come in their way. All the foregoing are prwcocial 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 FINCH ; SPARROW, etc.), which live altogether upon seeds, and a few others, lilce the king fishers, which catch fish, the great majority indulge themselves in a miscellaneous diet of both vegetable and animal materials. Some called ((soft-billed,0 and including most of our song-birds, except the finches, are mainly in sect 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 mam mals, 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 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 in numerable insects which they capture for them selves, 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, pea cock, guinea-foiml, duck, goose and various pigeons and partridges, that are among the most valuable of his animal possessions.
Distinctive Character of North American Bird-Life.— 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 resembes 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—not counting the seabirds common to both oceanic coasts— 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 duc.ks, geese, shore birds and birds of prey. When we examine the list of birds of the interior fields and wood lands the general resemblance to those of Europe remains close, while their divergence from those of South America is very stnking. In fact, Europe has only two families of small birds not also represented in North America— 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 con nected with both Europe and Asia; and that at times in the past the subarctic climate was comparatively warm—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 differ ences 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. — One of the most important features in orni thology 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 re.stricted 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.