J. A. ALLEN: BIRD MIGRATION. 15 



it would be available in winter, while those able to subsist 

 upon a more varied diet would be less affected by the 

 change of season and hence make shorter migrations. 



It is obvious that migratory birds, of whatever class, 

 which seek high latitudes for breeding stations, find there 

 the conditions most favourable for reproduction, as regards 

 not only food but the general environment. Otherwise 

 they could not have successfully persisted for possibly 

 millions of generations in the selection of a breeding station 

 at which they can live for barely a fourth of the year. 

 Migratory birds that breed in the temperate and colder 

 latitudes are wanderers for from seven to nine months 

 of the year ; while some change their residence by a journey 

 of. only a few hundred miles, others travel thousands of 

 miles, as do many Warblers, Tanagers, Swallows, 

 Flycatchers, and Shore-birds. It is therefore by no means 

 a mere figure of speech to call a bird's breeding station its 

 real and only home, for here it is not only a settled 

 resident for a definite period, but is occupied with the 

 most important function of its life, the reproduction of its 

 kind. 



As already said, the district selected by a large pro- 

 portion of migratory birds as a breeding station is 

 climatically uninhabitable to them beyond the short 

 period required for the duties of procreation. The 

 reason why they leave it is therefore not far to seek, and 

 it is hardly worth while to waste words over the question 

 whether the return to milder latitudes is due to a fall of 

 temperature, to the failure of the food supply, or to both 

 combined. Neither is it material to inquire whether the 

 species might or might not be able to withstand the 

 environment at the breeding station for a few days, or 

 even weeks, after the young of the year are sufficiently 

 mature to start on the long journey to a milder country ; 

 for, since the purpose for which the long journey to the 

 breeding station was undertaken has been accomplished, 

 why should they linger ? Much more time, however, is 

 usually taken for the autumnal journey than for the 



