BIRDS VERSUS INSECTS. 241 



ages of its host than the bird ? It is possible that, should 

 predaceous birds, insects and other enemies be insufficient 

 in holding the moth in check, the conditions in the course 

 of years, after great damage had been done, would be so 

 favorable for the multiplication of insect parasites that they 

 might check for a time the ravages of the moth. Experience 

 in many countries shows that, where other enemies of a 

 pest fail to hold it in check, insect parasites, starvation and 

 epidemic diseases sometimes put an end for the time being 

 to the plague. In addition to the destruction of parasites, 

 together with the host, some ichneumons and tachinid 

 flies are captured in the imago form by the flycatchers. 

 Birds also destroy some of the predaceous insects. This 

 is especially true of robins, jays and crows. Yet Pro- 

 fessor Forbes has shown that the destruction of such insects 

 by birds is not necessarily an evil.* The bugs which appear 

 to be most useful in the destruction of the moth are believed 

 to have some immunity from the attacks of birds by reason 

 of their pungent secretions. Crows, however, destroy cer- 

 tain of these bugs. 



Conclusions. 



Our present knowledge of birds as enemies of the gypsy 

 moth in Massachusetts points to the following conclusions : 



At least a dozen species are very useful in this respect ; 

 probably twenty-five others are more or less useful, when 

 the moths become very numerous, or when other species of 

 insects which these birds prefer are scarce. - f 



Birds destroy larger numbers of the gypsy caterpillars 

 when the canker-worms or other hairless larvae are scarce 

 than when they are plentiful. 



Although birds gather in localities infested by the moth 

 for the purpose of feeding on the caterpillars, they do not 

 flock to such infested localities in such numbers as they do 

 to regions infested by migrating locusts or by hairless cater- 

 pillars. 



Birds are particularly useful in preventing the increase of 

 small colonies of the moth, and in this respect are allies to 



* Bulletin No. 3, Illinois State Laboratory Natural History, November, 1880, 

 page 83. 



