THE JOURNEYINGS OF BIRDS. 325 



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 breed- 

 ing 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 sea- 

 son; 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 bam 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. 



