Origins of Migration 35 



tain 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 con- 

 gregate in such colonies during the breeding season, but it should not be over- 

 looked 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 south- 

 ern tier of states; others only reach to southern New England, while the bulk of 

 the species press on from northern New England to Hudson 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, 

 Phcebe 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. 



The theory that is, perhaps, most naturally suggested, and the one that finds 

 widest acceptance as explaining the facts, is that migration began in a search for 

 food. That is, the food supply becoming short in the vicinity of the home (a 

 bird's home is thus assumed to be the place where it rears its young, and may 

 therefore be quite different from the locality where it spends the remainder of 

 the time) they wandered away in search of food, returning again and again to the 

 home vicinity. These journeys were extended farther and farther, the birds re- 

 turning each nesting season, undoubtedly oftener at first, to or near the locality 

 where they were born. This process went on until their wandering became a 

 fixed habit, and finally in the countless generations of birds that have come and 

 gone, this habit has been crystallized into what we now call, for want of a better 

 term, the instinct of migration. 



This idea has been amplified and extended by Alfred Russel Wallace 

 (Nature, X. 459). He supposed that "survival of the fittest" has probably 

 exerted a powerful influence in weeding out certain individuals. He supposed 

 further that breeding can only be safely accomplished as a rule in a given area, 

 and that during a greater part of the rest of the year sufficient food cannot be 



