96 The Mammals of Colorado 
are owls, hawks, weasels, minks, foxes, coyotes, cats, badgers, 
skunks, and other animals, as well as snakes. 
Their food is mainly vegetable, though they never refuse 
meat, and will eat their own species when caught in traps. 
The vegetable food consists of the tender portions of grass, 
seeds, and grain, and in winter they gnaw the bark from 
the roots and trunks of trees and shrubs. 
Because of their abundance and prolificacy the voles are 
of considerable economic importance, and at times do much 
damage to crops and trees. In Europe and Asia invasions of 
armies of these creatures have been reported, the animals 
moving in large numbers across the country, devouring much 
of the field and garden crops which come in their way. 
No such invasions have yet occurred in North America, but in 
the Humboldt \^alley, Nevada, in 1907, one species was so 
abundant that their numbers were estimated to be 12,000 
to the acre in some of the alfalfa fields, and were doing great 
damage. 
In this country much harm may be done by the normal num- 
ber of voles present in a field or meadow. It has been calcu- 
lated that the amount of green vegetation eaten by an adult 
in the course of a year is from 24 to 36 pounds, so that the large 
numbers of them often present in a field can consume much 
grass and cause considerable loss to the farmer, for they not 
only eat the grass but they leave much of it lying on the 
ground, having eaten only the tenderest part. In winter 
they burrow into the haystacks in the field and do much 
damage there. In grain fields the harm begins as soon as the 
grain begins to sprout, for the tender shoots are cut down 
and eaten; when the grain is ripe or nearly so the stalks are 
cut down to get at the grain itself, and when it is harvested the 
shocks and stacks are attacked and the grain in them eaten. 
Voles can also do much harm in gardens of any kind by 
attacking the young growing plants 
