The phrase "field mice" typically refers to small animals that live in fields or grassy areas. They are tiny rodents that scurry around and build nests underground. Full definition
I had a similar reaction a few weeks ago when my cat brought in a live field mouse for me to play with. (theinspiredroom.net)
After feeding on an infected field mouse, a host deer tick will attach itself to a deer or dog before being introduced to a human host. (premierpetrelocation.com)