332 Recent Literature. [f^^ 



a failing food supply, while the spring movement is incited by the periodic 

 activity of the reproductive organs, resulting in the necessity for the 

 return of the species to the peculiar conditions and surroundings to which 

 for long ages it has been undergoing special adaptation — in other words, 

 to its home. "' This is not, however, necessarily the place of origin of the 

 species, which, in the case of many of our Warblers, Tanagers, Flycatchers, 

 etc., may well have been within tropical latitudes, which are now merely 

 their winter resort and not their home or place of reproduction. 



In the discussion of migration the great fundamental fact that the life 

 of animals, and especially of migratory animals like birds, is made up of 

 annual cycles, as is the life of plants, which have their fixed and determi- 

 nate seasons for flowering and fruiting, is generally overlooked. Atten- 

 tion was long since directed to this factor by Chapman, but it seems not 

 to have received the attention to which it is entitled. After referring to 

 the fact that many animals manifest a desire for seclusion during the period 

 of reproduction, and that "many species of tropical sea-birds resort each 

 year to some rocky islet, situated perhaps in the heart of their habitat, 

 where they may nest in safety," he continues: "This is not migration in 

 the true sense of the word, but nevertheless the object is the same as that 

 which prompts a Plover to migrate to the Arctic regions, and, be it noted, 

 is just as regular .... As in the case of a Warbler which nests in Labradpr, 

 they are all affected at nearly the same time by an impulse which urges 

 them to a certain place. This impulse is periodic and is common to all 

 birds. . . .It is evident, therefore, that external conditions have not created 

 this impul.se, though it is possible that in many instances they may have 

 governed its periodicity. On the contrary, its causes are internal. In the 

 case of the sea-birds, for example, dissection will show an enlargement of 

 the sexual organs and it is this physiological change which warns the birds 

 that the season of reproduction is at hand."^ And, it may be added, 

 prompts them to seek their accustomed breeding resorts, be they nearby 

 rocky islets or remote arctic or subarctic latitudes. We have here the 

 key to the impulse of the spring migration, of which the return migration 

 in the fall is the necessaiy complement, inasmuch as in most instances the 

 winter conditions of the breeding grounds of most species are prohibitive 

 of their continued residence therein throughout the year. 



How they find their way in their migrations is certainly remarkable and 

 implies wonderful gifts of which we have no intimate knowledge, but 

 enough, it would seem, fairly to remove the subject from the realms of 

 that complete mystery so many writers seem to take pleasure in involv- 

 ing it. In addition to keen powers of A-ision and a retentive memory, 

 which together enable them to distinguish lancbnarks, and a remarkable 

 sensitiveness to meteorologic conditions, they may also pos-sess a, to us, 

 mysterious sense of direction, as sho^^•n by the recent experiments of Dr. 

 J. B. Watson with Noddies and Sooty Terns. Dr. Alfred G. Mayer, Direc- 



1 Auk, X, 1893, p. 104. 



2 Auk, XI, pp. 13, 14. 



