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 (Euvanessa anttopa) 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 



