524 Journal of Agricultural Research voi. xxvti, No. & 
winters. The ranches were large, mainly from 500 to 1,000 acres each, 
and some with as many as 4,000 acres. Cultural methods were lax at the 
time, and ditch banks, fence rows, and roadsides were a tangle of dense 
vegetation that remained uncut throughout the year. 
Under this wealth of cover and with the rich food of alfalfa, the native 
meadow mice (Microtus montanus) became abundant during the fall of 
1906, but were little noticed. By the early summer of 1907 the fields 
were full of them and only half a crop was harvested in the June cutting, 
and in August still less, and over whole ranches none. In the valley 
below Lovelock, Nev., crops and orchards had been destroyed or ser¬ 
iously injured, and many of the fields were bare of vegetation in Novem¬ 
ber, 1907, and the millions of mice were digging deep into the ground for 
the alfalfa roots. In some of the fields there were literally thousands 
of mice to the acre. 
The money loss to farmers from mice in that part of the valley was 
conservatively estimated at a quarter of a million dollars. Mice were 
also reported as very numerous and destructive to crops in the Carson 
and Paradise Valleys and at other points farther up the Humboldt 
Valley, but these areas were not included in the estimates of losses. A 
similar outbreak of these mice was reported by the ranch owners in 
1899 and 1900, with almost as heavy losses as in the one cited. 
A poisoning campaign was instituted by the Biological Survey of the 
United States Department of Agriculture under the expert supervision of 
Stanley 1$. Piper. The farmers combined to distribute poisoned food 
until the numbers of mice were reduced to a point where the situation 
could be controlled by their natural enemies, the coyotes, foxes, bob¬ 
cats, badgers, skunks, weasels, owls, hawks (7, £), ravens, crows, magpies, 
gulls, herons, and shrikes, which had congregated in unusual numbers 
to feast on them. By the next spring the mice were scarcely more 
numerous than usual. Their food supply and protecting cover had 
been consumed during the winter and reproduction had diminished while 
their enemies multiplied. 
STUDY OF HABITS 
To understand more fully the underlying causes of such rapid increases 
in numbers of these rodents, which are referred to generally in the litera¬ 
ture dealing with field mice (j, p. 341-346; 6 , p. 292-299; 13 , p. 174-177; 
14, p. 403-406; 15, p. 273-276; 17 , p. 186-191; 18, p . 97-JOJ; P• 5 I 5~~ 
337 ), the Biological Survey has undertaken studies of the breeding and 
feeding habits of the common eastern representative of the group, Micro¬ 
tus pennsyhanicus , which with its subspecies covers over half of the 
Continent of North America. To recount these experiments in detail 
would be tiresome and needless, for the general results are sufficiently 
startling in significance and practical application. 
GENERAL HABITS OF MICROTUS PENNSYLVANICUS 
The Pennsylvania meadow mice are primarily meadow dwellers. 
They are capable, however, of adapting themselves to almost any situa¬ 
tion where a choice food supply is obtainable, and live in flooded marshes 
where they are forced to swim from one dry spot to another, cross small 
streams, or even range out over dry uplands in fields or orchards. They 
are active throughout the year; in winter, with thick fur coats, enjoying 
the restricted life under the deep snows in the north as much as their 
greater freedom with its increased dangers in summer. They dig nu- 
