Breeding nests are spherical constructions woven from grass or other plant material. A nest is approximately 13 centimeters in diameter and lined with a more downy material of fibrous plants. A nest may have one or more entrances near its base. Most commonly, the nest is built on the ground in a protected area such as within a shrub or beside a fallen tree; however, the mouse will occasionally place the nest aboveground within a shrub. It is not uncommon for a female to have ten to fourteen litters per annum, with a typical litter size of two to six individuals; however, litters of up to nine offspring can occur. Thus an annual production of forty to sixty young per female is normal. The Western Harvest Mouse is nocturnal, with particularly intense activity on very dark nights. This mouse is particularly resourceful, making use of the ground runways of other rodents. It is also a very agile climber. Its primary food source is seeds, but springtime dining is augmented with new plant growth. In June, July and August the mouse is known to consume certain insects, especially grasshoppers and caterpillars. Again resourceful, the Western harvest mouse stores seeds and other foodstuffs in underground vaults. Its many predators include the fox, weasel, coyote, hawk, snake and owl species.
