rowly defined, they are more or less clearly manifestations of the same influences and go to make up the sum total of this wonderful ebb and flow of bird life.
The origin, or perhaps better the origins, of this habit or instinct of bird migration is exceedingly obscure. Many theories have been advanced to account for it, but perhaps none has yet been offered that explains satisfactorily all its multitudinous phases. For instance, it has been suggested that migration is the result of the development or acquirement of the power of flight. That flight has had much to do in making long extended migrations easily possible no one can deny, but that it has been the cause is not logically evident, for certain mammals, as the bison and antelope, are to a limited extent migratory, and certain flightless birds, as the penguins and the great auks, are strictly so, or rather were in the case of the latter species which is now extinct.
According to Mr. F. M. Chapman ('Bird Studies with a Camera,' p. 194) 'the desire for seclusion during the breeding season' is a 'good and sufficient cause for the origin of bird migration.' He applies this theory especially to birds nesting in colonies in secluded spots, as the Ipswich sparrow which is known to nest only on Sable Island, off the Nova Scotia coast, the gannets (Sula bassana), which nest in the western hemisphere only on three islets in the Gulf of St. Lawrence, terns on Muskeget and Penikese, and the brown pelicans of the Indian River region of eastern Florida.
This theory may afford an explanation for the migrations of birds that congregate in such colonies during the breeding season, but it should not be overlooked that 'survival of the fittest' may have been an equally important factor in weeding out those individuals of such colonies that did not seek these secluded or isolated localities for breeding sites. These birds may at first have nested in scattered situations and have been driven by predatory animals or other causes to seek inaccessible locations, and seclusion and isolation may thus have been a resultant rather than a cause. It is also difficult to apply this theory to land birds. Take, for example, the warblers of the genus Dendroica. Some species barely reach the United States during the nesting season; a few stop in the southern tier of states; others only reach to southern New England, while the bulk of the species press on from northern New England to Hudson's Bay. If seclusion were the only point aimed at, it would seem that the warblers which pass farthest north to breed could have found it in the mountains of the southern and middle states as some now do. Again, certain species, as the cliff and barn swallows, phoebe and summer warbler, seek the vicinity of human habitations during the nesting season, and, moreover, have greatly increased in numbers since the country became thickly settled.