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 condi tions. 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—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 —perhaps 20 degrees north of the lati tude of Maine—because it finds there an aver ne mid-summer warmth like that of southern New England. Thus are constituted approx imately east-and-west districts, or °zones,* 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 Lalce to a point half way down the coasts of Hudson Bay and into northern Labrador. These dwell in . the *Arctic') Zone. South of it lies the narrow *Hudsonian)) Zone, which swings from the mouth of the Saint Lawrence along the south ern shore of Hudson Bay and northwestward to Alaska, including all of that peninsula. South of this the *Canadian° 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 °Tran sition)) Zone—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 Rocicies. The re
mainder of the United States east of the Rocicies is included in the °Austral* Zone, which is divided into an *Upper)) and a ((Lower* half, the latter embracing the South Atlantic and Gulf States. The Rocky Moun tains 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 ((region* 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 The gen eral subject of migration (q.v.) is treated else where;. but a few words may be pertinently added in this connection as to local peculian ties. 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 °residents.* 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 south ward in the fall, and visit the region of the Great Lakes., northern New York and New England dunng the cold weather, but rarely go farther south. These are the truc ((winter birds)) 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 ahnost all of the song-birds, or those most noticeable in sum mer, malces it seem to the uninformed north erner as if the whole bird-tribe had left the cotmtry.