AN ECONOMIC STUDY OF FIELD MICE: 



INTRODUCTION. 



The present paper deals with the habits of the common field mice a 

 of the United States, the conditions which often favor their enormous 

 multiplication, the natural enemies which aid in their repression, 

 and the agencies which farmers may employ to avoid losses by the 

 animals. Small as these pests are, they inflict enormous injury upon 

 the crops of the country. The aggregate loss to the farmers from 

 this source averages not less than three millions of dollars annually, 

 and in some years is much greater. The major portion of this loss 

 is preventable, and the object of this bulletin is to acquaint farmers, 

 orchardists, nurserymen, and others with the most practical pre- 

 ventive methods. 



Among the more interesting facts connected with wild animals 

 are the sweeping changes in the relative numbers of certain species 

 to be noticed from year to year in almost ever}^ locality. Species that 

 are abundant one season may be rare or entirely absent the next; or 

 they may gradually increase or decrease in numbers through a series of 

 years until disaster results from their overabundance, or the species 

 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 as of 

 miraculous origin. The belief that crickets, locusts, frogs, and even 

 mice sometimes fall from the clouds is still held in many countries. 



The careful observer, however, sees little mystery in the phenomena 

 mentioned. He has studied the general habits of animals — their 

 food, their powers of reproduction, their migrations, the checks on 

 their increase due to natural enemies, disease, and varying climate — ■ 

 and consequently he attributes sudden changes in their numbers to 

 known causes. In such changes he recognizes, especially, the influ- 

 ence of man, both direct and indirect, and his responsibility for inter- 

 ferences that greatly modify the operations of nature. 



° The term " field mice " applies equally well to several groups, or 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. 



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