THE MIGRATION OF INLAND BIRDS. 121 



prove to be the case that food was not sufficiently abun- 

 dant in the south for both its resident and migratory birds. 

 This certainly could not have been the case, and I believe, 

 therefore, that migratory movements, at the outset, were 

 very limited in extent, and consisted only of a few birds 

 at a time, which, seeking to avoid their enemies and have 

 undisturbed possession of a locality, pushed out from 

 their accustomed haunts for, comparatively speaking, a 

 few miles. The young of such pioneer birds would 

 naturally leave the neighborhood of their nest and return 

 to their parents' usual haunt with them ; but, on the re- 

 turn of another breeding-season, they would themselves 

 seek a nesting-place near where they themselves were 

 reared, and the older birds would go to the same nest or 

 nesting-place that they previously had occupied. This 

 is precisely what occurs now, year after year. JSTow, as 

 birds increased, century after century, the limits of this 

 northward movement would be extended, until it became 

 in time the journey of thousands of miles that it now is. 



Assuming, then, that migration arose for the dual 

 purpose of safe nidification and a certainty of sufficient 

 food, we are met by the ugly question, " Why do not all 

 the southern birds come north?" If, when the whole 

 avi-fauna was concentrated at the south, there was any 

 struggle whatever for favorable feeding- or breeding- 

 grounds, then, naturally, the weaker would go to the 

 wall, or, in other words, would be driven beyond the 

 limits of their accustomed habitat. These weaker birds, 

 taken together, having once formed the habit of visiting 

 certain localities at stated times for given purposes, or 

 being periodically forced to do so, they would vary in 

 their methods of reaching these localities, in their choice 

 of regions wherein to remain, and in the length of their 

 annual visit, just in proportion as their habits generally 



