GENERAL OBSERVATIONS AND SKETCHES AFIELD 311 



The Hermit Thrush as a Selective Factor 



The food habits of migratory birds, and their bearing as a 

 factor in the control of other organisms, especially insects, 

 form an interesting theme. Until these habits are thoroughly 

 known, we cannot hope to elucidate some of the fundamental 

 problems of evolution in insect forms. That birds exercise 

 a selective power and that this unconscious selection is a factor 

 in the modification of insects has long been maintained. Taking 

 one common bird hke the hermit thrush, we find that it con- 

 sumes a great number of insects. 



This thrush is a very common migrant in Michigan and 

 Illinois. It has the same general habits as the Swainson 

 thrush, with the exception that it is not so shy. It may be 

 seen silently moving about in the woods among the under- 

 brush, and when startled it has a tendency to fly straight until 

 it nearly reaches its selected branch, when it gives a sudden 

 sideways swoop and alights. Then, gently raising and lowering 

 its tail, one is treated to the low uttered "chuck." It rarely 

 sings during migration. 



In the spring it arrives between April twelfth and the middle 

 of May. On the return migration in the fall, it remains only 

 from the middle to the last of October. This species, like 

 many other migrants, is influenced somfewhat in its time of 

 arrival and departure by the condition of the weather. It 

 usually comes in great abundance. 



Forbes, in writing of the hermit thrush, says: "Considering 

 the fact that all these birds travel slowly the whole length of 

 the State, merely keeping pace with the advancing and retreat- 

 ing seasons, and also that the species is a very abundant one 

 at the period of migrations, it will be spen that its food has 

 great economical significance. There is reason to suppose 

 that these migrants, in passing north and south, follow year 

 by year about the same route; do not vary, that is, far to the 

 east or west. Consequently, occupying, as we do, a State that 

 lies in five and a half degrees of latitude, we can do much to 

 protect this species in its wanderings, or can, if we choose, 

 almost entirely eliminate the part of it passing over our territory 

 (Illinois). Twenty-one hermit thrushes were taken during 



