11 



fold by the birth of young; that though in winter there may be room for a 

 considerable number of birds in the southern stations, the natural spring 

 increase of population outgrows the supporting power of the land; and that 

 just at this critical time the whole northern temperate region is thrown 

 open to occupation with an abundance of food, the subject is mysterious 

 no longer. In fact, it is only by migration that it is possible to use the 

 supporting power of the temperate regions unless the birds fast or 

 hibernate through the winters, to neither of which the avian nature takes 

 kindly. 



Though food supply is the fundamental or originating cause of migra- 

 tion we must look for other and more immediate impulses for an explanation 

 of its methods to-day. Originally forced to and fro by hunger, the annual 

 movements now have become instinctive and take place before the situation 

 becomes acute, the actual hunger pinch felt, or the physical system 

 weakened by want. 



The extent of the migrations of the different species varies. A very 

 few species do not, in the true sense of the word, migrate at all. In other 

 species the more northern individuals only recede from their stations, 

 the southern remaining practically stationary, though in the majority of 

 Canadian species the whole body moves south. The bird of greatest 

 length of migration is doubtless the Arctic Tern, a bird that nests from 

 the gulf of St. Lawrence to the polar regions and winters as far south as the 

 Antarctic continent. 



The methods of migration are nearly as varied as their extent. Some 

 species drift along throughout the day from treetop to treetop, from wood 

 patch to wood patch, gradually working their way in the desired direction. 

 Others take long flights, some high in the air, others lower. Some travel 

 altogether by day; others travel at night and we are only aware of their 

 passage through accidental opportunities, their faint voices coming down 

 to us from overhead in the darkness, or by their sudden appearance about 

 us in the morning. They travel in flocks of single or mixed species, scattered 

 groups, or as individuals. 



Many species, if not all, follow more or less definite routes to and 

 from their breeding grounds and some go and return by altogether different 

 paths. Comparatively small bodies of water deflect some species from 

 their course, others unhesitatingly cross vast reaches of sea, indifferent 

 to nearby and convenient land passages that are made use of by closely 

 alUed species. In some species the older birds precede and in others 

 the males may precede the females. ^.^^ 



How birds find their way is still only vaguely understood, and indi- 

 viduals far out of their natural range and course show evidence of being 

 as hopelessly lost as any other animal would be on unfamiliar ground. 

 Certainly experience has much to do with it and undoubtedly young birds 

 are largely guided by the movements of their elders which, it can be 

 assumed, through previous experience, already know and can lead the way. 

 We can understand how birds can follow great landmarks — ^large river 

 systems, mountain ranges, or sea coasts in their journey, but no sense with 

 which we are familiar explains how some species return unerringly to lonely 

 oceanic islands over wastes of monotonous sea. It may be that they have 

 a special sense which aids them in orienting themselves. 



57172—2 



