WyYoMING Birpbs. 13 
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 con- 
sists in their eminently insectivorous habits and in the power 
possessed by them, in common with most other birds, of as- 
sembling quickly where food is plentiful. They thus form 
a sort of aerial police, whose chief function is to put down 
uprisings of injurious insects.” 
One fine Sunday im October; 1904, I saw.a. flock of 
Warblers about a few poplar trees near the river. They 
were feeding on swarms of mature aphids. I watched them 
at intervals all day. The flock seldom exceeded fifteen 
birds, mostly Blackpoll and Myrtle Warblers. Before night 
the swarms of insects that had been so numerous in the 
morning had dwindled so that it was rather difficult for me 
to secure a specimen, although the birds still found some. 
When I went there the next morning a single remaining 
bird was still finding a few, but I could not see a specimen, 
nor have I seen one there since.” 
“In 1905, I returned to my home at Wareham, Massa- 
chusetts, the first week in November, and found a flock of 
Myrtle Warblers busy hunting over the limbs and twigs of 
some apple trees and pear trees near my house. From the 
actions of the birds I concluded that they had discovered 
an outbreak of some pest, but at first I could see nothing on 
the twigs that they were inspecting. By watching them with 
a glass, however, I soon saw exactly where they were find- 
ing food. I saw that they were feeding on a minute cicada- 
shaped, black insect. This, indeed was the only species of 
insect I could find on those trees. Three of these insects 
were secured and two were sent to Dr. L. O. Howard, Chief 
of the Bureau of Entomology at Washington. He identified 
them as the imago of the pear tree Psylla, a pest which has 
