Mice are primarily herbivore animals, although opportunity may lead them to eat almost anything, especially in extreme situations where nothing more suitable is available. This is especially true of city mice, which have adapted to feed on human food scraps and become partially dependent on garbage and human disposals to feed on a regular basis.
In the wild, most mice eat whatever vegetation is available, including fallen fruits and seeds. Depending on the season and the area where they live, these can include anything from tree bark to bulbs or roots, oats, corn or even potatoes and black beans. In the winter, mice will dig through the snow in order to reach grass or sprouts.
Certain mice, like the white-foot species, are omnivorous and will eat insects or meat scraps, if available. The grasshopper mouse is almost exclusively carnivorous. Not only does this species eat meat, but it is also excellent hunters, feeding primarily on grasshoppers, small insects and even some small mammals or other rodents.
Mice that live in the countryside, near farms or crops, often venture closer and pick on garbage or food disposals. If corn, wheat or other types of "over-the-ground" crops are being grown, they are likely to feed on that. Since they can squeeze through very small openings, they are also likely to feed on seeds, bulbs or crops that are in storage or being air-dried.
Pet mice should be fed commercial pelleted food or clean seeds, which contain all needed nutrients for optimal health and care. The diet is best complemented with fresh vegetables, as mice are sometimes resistant to drinking water and the veggies will provide enough liquids to keep them hydrated.
Certain foods, including some common vegetables, are poisonous to mice and should never be given to pet mice, especially baby ones. Avocado is especially toxic, but mustard seeds, candy, coffee beans and chocolate can be equally dangerous, depending on the amount and concentration.