FOOD OF FIELD MICE. 13 



FOOD HABITS. 



Owing to its finely chewed condition, exact determination of the 

 food of rats and mice from stomach examinations alone is very diffi- 

 cult. This is especially true of the species of the genus Mierotus, 

 whose molars are well adapted for grinding. So complete is the 

 mastication and subsequent maceration of weed and grass seeds, as 

 well as roots and grains, that the cell structure is often destroyed. 

 A fair idea of the food can be gained, however, by a study of the 

 animal's environment, by the color of the stomach contents when 

 bark has been eaten, the odor of wild onions when present, the pres- 

 ence of starch grains revealed by the microscope, or the character of 

 the few perfect vegetable cells that remain. The bits of steins, blades 

 of grass, and leaves of other plants left scattered along the run- 

 ways are important evidence, as also are the contents of the caches 

 of food in the burrows. 



In summer the principal food of these mice is green vegetation and 

 unripe seeds of grain and grasses. As the season advances, ripe 

 grain and seeds take the place of the immature ; and in winter bulb- 

 ous and other roots are in part substituted for steins and leaves. 

 When convenient, and green vegetation is lacking, the bark of trees 

 and shrubs becomes a staple food. It is mainly in winter that apple 

 orchards and young forest trees suffer from attacks of mice. It is 

 generally supposed that such attacks are due to the absence of ordi- 

 nary food; but this is not always the case, for depredations often 

 occur during mild, open winters when food abounds. Instances also 

 of summer girdling of trees are well attested. 



Examinations of stomachs of field mice show that, besides stems, 

 leaves, and seeds of grasses and sedges, the animals eat nearly all 

 kinds of bulbs, tubers, and roots, and occasionally animal food such 

 as snails and crayfish. Outside the list of grains, vegetables, grasses, 

 roots, and barks, which are generally known as staple food of field 

 mice, they have been found eating strawberries and other fruits; 

 roots of wild morning-glory (Convolvulus sepium), sweet clover 

 {Melilotus alba), and cat-tails (Typha latifolia) ; seeds of iris and 

 primrose (Primula parryi) ; bulbs of tulip, hyacinth, and wild onion; 

 and the tubers of the Jerusalem artichoke (Helianthus tuberoms). 



The quantity of green vegetation eaten by a single adult field 

 mouse in the course of a year has been calculated at from 21 to 36 

 pounds. When one considers in connection with this estimate the 

 great numbers of these animals in our meadows, swamps, and forests, 

 the total quantity of food consumed by them appears so enormous as 

 apparently to exceed the productive capacity of the soil. A thou- 

 sand mice in a meadow would require at least 12 tons of grass or 

 other green vegetation to maintain them for a year. 



