122 USEFUL BIRDS. 
tain plant lice; and two years previously I had published a 
list of thirty-four species which feed upon plant lice. It is 
a widely known fact in Massachusetts that practically all of 
the resident and migrant Warblers eat the birch plant louse. 
It is only necessary for one to find a locality where these 
insects are numerous if he wishes to make sure of finding in 
their seasons about all the Warblers that breed in that region 
or migrate through it, and also many other birds not ordi- 
narily found among the birches. Trees are seldom killed 
by plant lice; but they are often seriously weakened, their 
fruitage lessened, and their growth greatly retarded by the 
attacks of these prolific creatures. Undoubtedly the plant 
lice of the birch would greatly reduce the annual crop of 
birch wood and lumber were it not for the manner in which 
their increase is checked by birds. A list of thirty-eight 
species of birds that have been found, either by myself or 
my assistants, feeding on birch plant lice, is appended : — 
Downy Woodpecker. Chickadee. 
Northern Flicker. Scarlet Tanager. 
Chimney Swift. Red-eyed Vireo. 
Ruby-throated Hummingbird. Yellow-throated Vireo. 
Wood Pewee. Black and White Warbler. 
Least Flycatcher. Myrtle Warbler. 
Purple Finch. Parula Warbler. 
Rusty Blackbird. Yellow Warbler. 
Red-winged Blackbird. Black-throated Blue Warbler. 
Baltimore Oriole. . Magnolia Warbler. 
American Goldfinch. Chestnut-sided Warbler. 
Slate-colored Junco. Northern Yellow-throat. 
Chipping Sparrow. Black-throated Green Warbler. 
White-throated Sparrow. Black-poll Warbler. 
Field Sparrow. Oven-bird. 
Bobolink. American Redstart. 
Towhee. Catbicd. 
Rose-breasted Grosbeak. White-breasted Nuthatch. 
Indigo Bunting. American Robin. 
Some of the evidence from which my conclusions were 
drawn regarding the economic relations existing between 
birds on the one hand and plant lice and hairy caterpillars 
on the other, is here presented, that the reader may have an 
