ANNIVERSARY ADDRESS 191 



For the return moveinents in spring similar reasons 

 have often been assigned, though of a converse nature ; 

 the climate proves excessively hot, proper food supplies 

 proportionate to the increase in numbers are lacking; 

 in short, the birds find the cooler regions more suitable 

 during the period of reproduction. But once again we 

 must remember that many species breed under a tropical 

 sun, and that of these certain can hardly be said to 

 migrate in the strict sense of the word — if indeed they 

 do so at all — while those that attain the highest latitudes 

 of the Northern Hemisphere m summer are often found 

 to overpass their fellows on migration, and to reach the 

 most southerly climes in winter, where the heat at least 

 would hardly incommode them in summer. Moreover, 

 it is evident that many individuals press on beyond the 

 limits which seem to supply all that could be desired 

 for nesting purposes ; but to understand this we need 

 only consider that an area, which would satisfy the 

 requirements of a very considerable pojmlation, is found 

 lacking when demands are made upon it in excess of 

 its productive powers. For the return migration then 

 it appears we must seek further cases which are not 

 at present evident. 



To sum up, we may be justified in assigning the main 

 cause of autumnal migration to some deficiency in the 

 food suppl}^ while it is evident that this failure, or partial 

 failure, will occur at different periods of different years, 

 in accordance with the severity of the weather ; and 

 thus we may account with ease for the earlier departure 

 of birds in certain seasons. In other words, " Cycles of 

 Weather," if such there be, ma}^ be found with matured 

 experience to be coincident with " Cycles of Migration." 



Such being, therefore, the causes predisposing to 

 migration, it lemains to consider in what direction the 

 movements will presumably take place, and how far our 

 theories are in accordance with ascertained facts. The 



