188 ANIMAL COMPETITORS 



made their home in his pigeon-house. Think- 

 ing they were after the pigeons he shot the 

 male and the next day trapped the female. On 

 investigation he found four young owls in the 

 nest, together with the remains of ten pocket- 

 gophers. He immediately released the cap- 

 tured mother, with his apologies. This owl is 

 so regular in its daily capture of these pests 

 that it might well be named the gopher-owl. 



Serpents devour mice and gophers. Snakes 

 must be included among the natural enemies 

 of field-mice. The larger bull-snakes (Pituo- 

 phis), blacksnakes (Callopeltis), and rattle- 

 snakes (Crotalus) feed largely upon rabbits, 

 prairie-dogs, pocket-gophers, and ground-squir- 

 rels, as well as different species of rats and 

 mice. Blacksnakes and bull-snakes probably 

 kill more field-mice than the others ; but black- 

 snakes destroy also a considerable number of 

 nestling birds and birds' eggs, so that part of 

 their beneficial work is offset by this injurious 

 habit. A nurseryman in Pennsylvania reports 

 that he secured immunity from mice in his nur- 

 sery by turning loose in it 50 blacksnakes. 



The Pacific bull-snake (Pituophis catenifer), 



