31 



Already, because perhaps of the excessive destruction of the 

 enemies of birds and mice in some parts of the United States, 

 these little rodents are causing serious loss. The greatest out- 

 break in this country of which I have seen definite records 

 occurred in the Humboldt valley, Nevada, in 1907-08. Here 

 four great ranches suffered an estimated loss of $86,500, and 

 the damage in the immediate region was estimated at about 

 $300,000, with injury less severe extending up the river and its 

 tributaries. 1 



Field mice when properly held in check by their natural 

 enemies perform several useful offices. Then, according to 

 Rhoads, the food of the common meadow mouse (Microtus 

 pennsyhanicus pennsylvanicus) consists mainly of rushes, sedges, 

 salt grass and other coarse grasses and weeds, and from 70 to 

 80 per cent of the whole number of field mice ordinarily live in 

 bogs and low moist lands, where they do little if any harm, 

 while those on uplands nearly all confine their foraging to 

 fence rows, brush patches and neglected places, rarely eating 

 any except waste grain. 2 



Thus when in normal numbers they do good rather than 

 harm, by converting worthless rushes, grains and weeds into a 

 supply of food for fully two-thirds of the natural enemies of 

 birds which, by means of the superfluous mice, easily taken, 

 are fed sufficiently to prevent them from becoming too de- 

 structive to birds. Exterminate mice and the problems of the 

 farmer and those of the economic zoologist would be wonder- 

 fully increased, but exterminate the natural enemies of mice — 

 then the deluge. The only effective artificial method of meeting 

 the great invasions of mice that occur through lack of natural 

 enemies is to use poison, which is likely to be destructive to birds 

 and other animals as well as mice. Therefore it is suicidal to 

 destroy too many shrikes, crows, hawks, owls, herons, bitterns, 

 gulls, foxes, skunks, weasels and other creatures which feed on 

 mice, even though they may feed to some extent on birds also. 



The two forms of mice which are, perhaps, the most widely 

 disseminated, and prolific of all native mice are the common 

 meadow mouse and the deer mouse. 



1 Piper, Stanley E.: U. S. Dept. of Agr., Farmers' Bull. No. 352, The Nevada Mouse Plague 

 of 1907-08, 1909, pp. 9, 10. ' 



3 Rhoads, Samuel N.: Mammals of Pennsylvania and New Jersey, 1903, pp. 98, 99. 



