ANNIVERSARY ADDRESS 189 



their winter homes. The object of this may be partly 

 the protection which greater numbers afford ; partly the 

 comparative ease of procuring food when many individuals 

 are bent on the same quest, and notify the results to 

 their neighbours ; partly the guidance given by the 

 stronger to the weaker of the band. But here we must 

 pause to note that the stronger birds are not necessarily 

 the older ; on the contrary, facts go to show that the 

 young not only travel by different routes to their elders, 

 but usually leave the land of their summer sojourn before 

 the adults, though in such cases as that of our common 

 Cuckoo the reverse is very noticeable. The longer 

 winged individuals of a species appear often to migrate 

 to a greater distance than their fellows, whether it be 

 by starting at an earlier date, or by outstripping them 

 owing to their greater mechanical powers of flight. 

 This being so, it is far from universally tiue — if true 

 at all — that the older birds act as leaders to those of 

 tender age, a theory which has been broached by more 

 than one wiiter on the subject. And in truth how 

 the bands find their way is a question that as yet 

 remains unanswered. 



Next in order we must consider the reasons which 

 induce birds thus to change their quarters with such 

 great regularity. 



There can be little doubt that the main cause of 

 the movements is the growing scarcity of the food supply, 

 though climatic conditions must certainly be taken into 

 account, the more so as the abundance or lack of nutriment 

 depends in the long run on the season and the weather. 

 The rigours of an Arctic or an Antarctic winter would 

 no doubt sooner or later be sufficient to drive all wino-ed 



o 



creatures to warmer regions ; but it is plain that they 

 do not remain in their breeding haunts until the tem- 

 perature is of itself sufficiently low to expel them. On 

 the contrary it is the gradually increasing cold of 



