Part II.] NATURAL ENEMIES OF BIRDS. 237 



Field Mouse or Meadow Mouse (Microtus pennsyhanicv^ penn- 



sytvanicus). 



Rhoads says that under normal conditions only about 5 per 

 cent of the food of this species consists of animal matter. It de- 

 stroys some insects and eats dead animals, birds and eggs. Dr. 

 George W. Field informs me that this species interfered with 

 some experiments in rearing bobwhites undertaken by the 

 Massachusetts Commission on Fisheries and Game, as it de- 

 stroyed many eggs. Whenever the numbers of field mice 

 increase, so that their food supply is threatened, they will turn 

 quickly to animal food, eating dead animals, destroying live 

 ones and even killing and eating one another. I am not aware 

 that the effect produced upon bird life by these swarms of 

 mice has been investigated, but it is easy to understand that 

 it must be very serious. 



Deer Mouse {Peromyscus leucopv^ noveboracensis) . 

 The habits of the deer mice, now separated by the system- 

 atists into several forms, are somewhat like those of the garden 

 dormouse of Europe. Although quite at home on the ground 

 the deer mice or white-footed mice are very partial to trees, 

 and often make their nests in hollow trees or in birds' nests, 

 no doubt in some cases depriving birds of their homes. Dr. 

 Abbott says that they are skillful hunters of birds' nests, and 

 that he has known them to rob the nests of the robin, song 

 sparrow and chewink, and asserts that he has seen young 

 robins, nearly fledged, killed by them. The mice returned to 

 the nest when the parents were absent and ate the young 

 birds.^ Mr. M. A. Walton of Gloucester, Massachusetts, wrote 

 me in detail that he had known these mice to destroy birds on 

 many occasions. The birds killed were young or injured. When 

 he placed nests with young birds in his cabin in the woods for 

 safety over night he found invariably that they were eaten by 

 these mice unless protected by wire netting. He asserts that 

 the mice robbed the nest of a white-throated vireo near by 

 and brought one of the dead young into the cabin. Stone and 

 Cram assert that these mice appropriate the nests of birds in 



^ Abbott, Charles Conrad: A Naturalist's Rambles about Home, 1885, p. 70. 



