il 
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 
ee through the winters, to neither of which the avian nature takes 
ndly. 
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 anuual 
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 
usin themorning. 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 
allied 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 
