W ECONOMIC STUDY OF FIELD MICE. 
INTRODUCTION. 
The present paper deals with the habits of the common field m 
of tlif United Stat*'-, the conditions which often favor their enormous 
multiplication, the natural enemies which aid in their repression, 
ami the agencies which farmer- may employ t«> avoid losses by tin' 
animal-. Small a- these pests are. they inflict enormous injury upon 
the crops of the country. The aggregate loss to the farmers from 
tin- source averages not less than three millions of dollar- annually, 
ami in some year- i- much greater. The major portion of this loss 
i- preventable, and the object of this bulletin is to acquaint farmers, 
orchard ists, nurserymen, and others with the most practical pre- 
ventive method-. 
Among the more Interesting facts connected with wild animal- 
are the sweeping changes in the relative numbers of certain species 
to l»e noticed from year to year in almost every locality. Specie- that 
are abundant mie season may he rare or entirely absent the next: or 
they may gradually increase or decrease in numbers through a series of 
year- until disaster results from their overabundance, or the sp 
becomes practically extinct. Sometimes wild animals increase in 
numbers so suddenly that the change has been likened to a tidal 
wave, and ignorant people have regarded the invasion a- of 
miraculous origin. The belief that cricket-, locusts, frogs, and even 
mice sometimes fall from the clouds is -till held in many countries. 
The careful observer, however, sees little mystery in the phenomena 
mentioned. lie ha- studied the general habits of animal — their 
food, their powers of reproduction, their migrations, the check- on 
their increase due to natural enemies, disease, and varying climate— 
and consequently he attribute- sudden changes in their numbers to 
known causes. In such changes he recognizes, especially, the influ- 
ence of man. both direct and indirect, and hi- responsibility for inter- 
ferences that greatly modify the operation- of nature. 
a The term "field mice" applies equally well t<> Beveral irrinips. <>r genera, of 
mice which occur in cultivated areas and meadows of the United States, but in 
this paper it is restricted to the most widely known group, the genus Microtus. 
