16 "USEFUL BIRDS. 
on the leaves, the borers that attack the twigs, and the insects 
that destroy the blossom or the fruit, all probably, when in 
normal numbers, exert a useful influence by a healthful and 
necessary pruning, which at least does no injury to the tree. 
It is only when these insects increase abnormally in numbers 
that they seriously injure or destroy many vigorous plants 
and trees. During such outbreaks birds often come to the 
rescue of the trees. Birds feed very largely on such insects, 
and by keeping down their excessive multiplication perform 
a great service in the economy of nature. 
Here the keen senses and remarkable flight powers pos- 
sessed by birds aid them in concentrating their forces imme- 
diately when and where they are most needed. The rule 
will bear repetition here that, other things being equal, birds 
will take such suitable food as is most plentiful and most 
easily obtained. This is especially true of the feeding of 
birds on insects, although there are some insects that are so 
protected by prickly spines or acrid secretions that few birds 
will eat them. Such are the caterpillars of the mourning- 
cloak butterfly (Huvanessa antiopa) and the imagoes of the 
Colorado potato beetle (Doryphora decemlineata) . 
Birds are quick to assemble wherever in the woods the 
disappearing foliage denotes the presence of great numbers 
of destructive caterpillars, or where ‘patches of dead and 
dying grasses indicate that grubs are destroying the grass 
roots on meadow or prairie. Birds flock to such places to 
feed on the easily procured insects, and so take a prominent 
part in repressing such insect outbreaks. This is so well 
known as to be worthy of only passing mention here, were it 
not to inquire whether the birds that assemble in such locali- 
ties do not neglect their normal and special work of hold- 
ing in check certain species elsewhere. If the Robin, for 
example, which feeds normally on such ground-frequenting 
insects as white grubs, cutworms, grasshoppers, March flies, 
and ground beetles, goes to the woods to feed on caterpillars, 
as is sometimes the case, does it neglect to devour any one 
of the insects on which it usually feeds, and so give this 
insect a chance to increase? If so, it would be merely sup- 
pressing one outbreak and permitting another. But birds 
