114 
BIRDS 
water and food. A probable cause of migration is the failure 
of food supply as cold weather comes on in the fall. 
94. Economic Importance of Birds. — The chief food of 
birds is insects, such as plant lice, larvae of beetles, butterflies, 
Figure 102 . — Young and Adult Chestnut-sided Warbler. 
moths, borers, etc. The chickadee, for example, feeds on 
plant lice as well as other foods; the downy woodpecker 
feeds on codjing moths and borers; the nuthatches and brown 
creepers feed on insects and insect eggs that are hidden in 
crevices and under loose pieces of bark. Other useful birds 
are the song-sparrow, chipping-sparrow, robin, bluebird, 
