harvest mouse

from The Century Dictionary.

• noun A very small murine rodent or mouse, Mus minutus, abundant in Europe, nesting in grain, and therefore specially observable in harvest-time. It is one of the very smallest of mice, being about 2¼ inches in length, with a tail nearly as long.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A species of the sub-family Murinae (Old World rats and mice).

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun small reddish-brown Eurasian mouse inhabiting e.g. cornfields
• noun any of several small greyish New World mice inhabiting e.g. grain fields