FIELD BOOK OF MAMMALS 



their presence by well-defined runways through the grass. 

 These Mice may be anywhere, provided there is sufficient 

 grass and low-growing foliage for food and cover, but the 

 best places to look for them are the extensive growths of 

 rank, heavy grass where there is sufficient moisture to main- 

 tain a meadow the year around. Here Microtus is found in 

 large colonies and the runways may be traced for long dis- 

 tances. 



These runways are easily recognized as the highways of 

 small mammals. To form them the Mouse cuts and deflects 

 the grass to form a clear passage or tunnel, and the floor is 

 kept clean of obstructions. Characteristic pellets, the mouse 

 droppings, are a feature of the runways, and are usually con- 

 centrated at intervals, often near the mouth of a burrow. 

 Here and there along the runways are holes leading down to 

 subterranean burrows and runways where the animals have 

 their nests. Often the runways make an obvious use of the 

 natural features of the terrain and extend along the sides of 

 logs, the lower rails of fences, etc. While most of the activities 

 of the colony are centered about the runways, individuals 

 may be trapped where there are no runways, showing that 

 the Mice are not entirely dependent upon them. 



The runways afford shelter from enemies and Meadow 

 Mice are more or less active throughout the day. One 

 frequently sees a Mouse darting along these tunnels in the 

 grass, and during haying time numbers are caught above 

 ground. Some species have grass nests for summer occupancy 

 at the surface or beneath flat rocks, pieces of bark, boards, 

 etc. Most of these Mice, however, retreat underground 

 when not feeding. Probably Meadow Mice are most active 

 at night, even in the case of those species which are seen 

 during the daytime, for traps seem to catch the most between 

 the hours of sunset and sunrise. 



The forms of the pennsylvanicus group have a very wide 

 geographical distribution, but nearly everywhere throughout 

 this range the preferred environment for pennsylvanicus and 

 its relatives is heavy grass-lands. The forms of most of the 

 other groups of species in the subgenus Microtus may be sought 

 in a like environment. The high-altitude species like nanus 

 live in beautiful mountain meadows and parks, and some of 

 the mountain species go above the tree limit and are found in 



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