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), 
