16 INTRODUCTION. 
were not so sharp-sighted as the ornithologists of to- 
day. It is not within the scope of this volume to 
enter into the varied details of the migration of our 
birds. It is a moderately uniform movement from 
points widely distant, and the question of a food- 
supply is the principal incentive. When migration 
commenced, and why, cannot be determined. Geo- 
logically, probably, it is of recent date,—that is, 
brought about by changes wrought during the 
growth, climax, and disappearance of the Glacial 
Period; but of course all this is conjectural. We 
cannot conceive of migration in a climate without 
seasonable changes, unless an effort to escape enemies 
during the breeding period should induce a general 
exodus from the ordinary haunts; but these enemies 
would soon be able to follow unless a climatic bar 
shut them out. Coming down to details in the mat- 
ter, and considering individual species rather than 
birds ex masse, it will be found that many birds that 
migrate in easy stages are very deliberate, and when 
they find comfortable quarters remain until a change 
of weather warns them to proceed. This is not 
noticeable to any degree in the spring-tide northward 
journey, but is very decided in the autumn return 
trip along the Atlantic coast, and presumably equally 
true of the interior. Early or late in September, as 
the case may be, the woods will fill up with warblers. 
Even the trees along the village street and the bushes 
in town gardens will be visited by a throng of dainty 
birds, some conspicuously colored, but mostly plain 
and likely to pass unnoticed, unless their ceaseless 
activity calls attention to them. Often they do not 
