FOOD OF WARBLERS 31 



each young bird requires fully half its own weight of insects each 

 day. As the young are fed very largely on caterpillars, and as 

 they are reared at a time when these insects are most plentiful 

 there is no doubt regarding the restraining influence exerted upon 

 the increase of such insect life throughout the North Temperate 

 Zone by a family of birds so abundant and widely distributed as the 

 Warblers. The usefulness of these birds in migration consists in 

 their eminently insectivorous habits and in the power possessed 

 by them, in common with most other birds, of assembling quickly 

 where food is plentiful. They thus form a sort of aerial police 

 whose chief function is to put down uprisings of injurious insects. 

 Such insects are of little importance except where they appear in 

 abnormal numbers. Wherever this occurs a counter-check is 

 needed, at once, lest by the geometrical progression of their 

 increase they overwhelm all opposition and sweep everything 

 before them. The migrating Warblers form such a counter-check. 

 They sweep over the country always on the watch for an abundant 

 food supply. Wherever food is plentiful the birds gather. Find 

 a great swarm of young caterpillars or birch plant-lice in the 

 spring and there you will find, in their seasons, practically all the 

 Warblers that pass through that region. 



The reduction of the numbers of insects by migrating Warb- 

 lers may be illustrated by a leaf or two from my own experience. 

 In the spring of 1903, an old field in Concord, Massachusetts, 

 grown up to birches, was much infested by plant-lice. Although 

 the spring flight of Warblers was small, these birches were fre- 

 quented by them. In the fall migration the birch field was again 

 the gathering place of Warblers, although elsewhere in the woods 

 the flight of birds was so meagre as to be hardly apparent. In 

 1904 the aphids were somewhat reduced in number, but the birds 

 followed them up, as in the previous year, until, late in October, 

 most of the plant-lice had disappeared, and the Myrtle Warblers, 

 the latest migrants, leaving the birches, attacked other plant-lice 

 on the wild apple trees. Since then comparatively few birch plant- 

 lice have been seen in the field. This may have been partly due 

 to the action of predaceous insects, parasites, or to adverse meteor- 

 ological conditions, but the effect produced by the birds was very 

 marked. 



One fine Sunday in October, 1904, I saw a flock of Warblers 

 about a few poplar trees near the river. They were feeding on 

 swarms of a mature aphis. I watched them at intervals all day. 



