Wyoming Birds. 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 in 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. H'e identified 

 them as the imago of the pear tree Psylla, a pest which has 



