THE MIGRATION nr• BIRDS. .More conspicuous and interesting. and quite as difficult to explain, are the migrations of the birds, which have been the theme of poetry, homily, and fable, as well as a baffling subject of inquiry. ever since men began to notice the ways of animals.
Most persons have a vague idea that the habit of yearly Migration among birds is uni form and universal; but this is not so. Most birds do not migrate at all. and among those that do great diversity exists, so great that the custom seems almost an individual rather than a raeial one. The whole body of ratite birds— ostriches, rheas, cassowaries, and the like—are non-migratory. The fish-eating sea-fowl make no more of an annual migration than is necessary to escape from the Re and darkness of their most polar haunts to where there may be open sea. These are wanderers rather than migrants. Gulls and terns, geese. ducks, and the wading marsh and beach birds are in the main migra tory, and include some of the most remarkable examples. Of the game-birds fewer are real migrants, but here again a few notable exceptions exist, of which one of the most familiar is that of the common European quail, which has been taken so numerously for centuries on both sides of the Mediterranean, and whose migratory flocks still feed travelers wandering in the wilderness of Sinai. The pigeon tribe is sedentary as a rule, also, yet one of its species— the passenger pigeon of North America—has be come the very type and exemplar of a migratory bird. Many, but not all, birds of prey regularly migrate, hut it is a question whether they do not, in most cases, accompany the movements of the smaller birds rather than travel of their own impulse. Parrots are almost wholly non migratory. It is not, then, until we have passed twenty-one of the twenty-three classified orders of birds (with the exceptions above noted) that we come to those groups—the piearian and pas serine birds—in which the custom of seasonal migration is a prominent characteristic. These are. to he sure, the most numerous as well as the most highly organized orders: yet a large number even here do not migrate at all from tem perate regions, but form a 'resident' or 'partially migrant' population in all moderate latitudes, where they remain all the year round. On the whole, the large majority of the total list of the birds of the world are non-migratory to any con siderable degree.
When we examine the minority which does an nually alternate between southerly winter and northerly slimmer residences, many curious facts are discernible. First, it is noticeable that all migratory birds belong to the colder latitudes of the globe: and, on the other hand, that those groups which are wholly non-migratory represent the primitive types—birds whose ancestry goes back to times when a comparatively warm cli mate prevailed over the now nnbearably cold and sterile polar regions. In general. two-thirds of the birds of the middle temperate zones, both north and south, are migrants, and the total is a very small part of the entire avifauna of the world. Taking up the character of the migratory birds as a class, it appears, first, that they arc such as either subsist wholly or mainly on soft bodied insects, larva-, worms, and the like, or give their young such fare: second, such as gain their living from fresh or brackish waters or mud, which is likely to freeze; and third, such as fol low small birds in order to prey upon them. It is also significantly true that they represent fami lies whose mass and affiliations are found in the tropics, in many cases only one or two species being known elsewhere. Europe's single cuckoo, our single (Eastern) humming-bird, our few tanagers. orioles, and the like, are familiar and striking examples of this fact. On the other hand, the non-migratory or 'resident' birds of the temperate zones belong to families mainly dis tributed outside the tropics, and separable, broad ly speaking, on other grounds. This state of things points to the explanation that the extra tropical parts of the world, depopulated of birds by the cold, ice, and excessive rains of the Pleistocene or 'Glacial' period, were restocked from the crowded intertropieni preserves as fast as the amelioration of the climate permitted plants and animals to occupy the temperate and sub-aretie regions: and that the reactive effect of the new country steadily checked colonization by selecting only those species adapted or adap tive to the new conditions. In this light, the seasonal migration of birds must he viewed as an annual excursion, constantly repeated by certain species that have the habit land not by others), outward from equatorial regions to a greater or less distance poleward.