58 ORIGIN OF MIGRATION. 
Origin of Migration.—Why do birds migrate? It 
is true that in temperate and boreal regions the return 
of cold weather robs them of their food, and they retreat 
southward. But many, in fact most, birds begin their 
southern journey long before the first fall frost. We 
have seen that some species start as early as July and 
August.. Furthermore, there are many birds that come 
to our Gulf and South Atlantic States to nest, and when 
the breeding season is over they return to the tropics. 
Surely, a lower temperature can not be said to compel 
them to migrate. Even more remarkable than the south- 
ward journey in the fall is the northward journey in the 
spring. Our birds leave their winter homes in the tropics 
in the height of the tropical spring, when insect and vege- 
table food is daily increasing. They leave this land of 
plenty for one from which the snows of winter have 
barely disappeared, often coming so early that unseason- — 
able weather forces them to retreat. 
I believe that the origin of this great pilgrimage of 
countless millions of birds is to be found in the existence 
of an annual nesting season. In my opinion, it is exactly 
paralleled by the migration of shad, salmon, and other 
fishes to their spawning grounds, and the regular return 
of seals to their breeding rookeries, 
Most animals have an instinctive desire for seclusion 
during the period of reproduction, and when this season 
approaches will seek some retired part of their haunts or 
range in which to bring forth their young. Salmon may 
travel a thousand miles or more from the ocean, and, 
leaping the rapids or other barriers in their way, finally 
reach the headwaters of some river where their eggs 
may be deposited in safety. Seals migrate with regu- 
larity to certain islands, where their young are born. 
Even our domesticated Hens, Turkeys, Ducks, and Pea- 
fowl, if given freedom, will travel a greater or less dis- 
