Baby field mice are nursed by their mothers for the first three weeks of their lives. After this, they eat the field mouse's usual diet of seeds, nuts, roots, cereals, fruit, insects and other small invertebrates. To aid in digestion, the mice will eat their own feces.
Because of their food habits, the beneficial aspects of field mice are mixed. On the one hand, they bury seeds in the ground that they don't return to dig up later. These seeds can germinate into new plants or trees. On the other hand, they dig up and eat seeds that have been planted by farmers or gardeners and reduce crop yield. They also damage seedlings and eat grasses so intensively that the grasses can't recover.
Field mice are born and raised in systems of tunnels that are excavated around tree roots. These tunnels or burrows might also have been the homes of other kinds of animals and have been taken over by the mice. Adult mice might live in these tunnels communally. Some "rooms" in the tunnel are set aside for food.
Field mice might also move in the homes of human beings during the colder months and eat the food they find there.