This was the first thing I thought of. We have two fat, lazy house cats, and we will never ever have mice. I grew up on a horse farm. We had Four cats that lived in the barn (12 horse barn) for the exact same reason. They were legitimate badasses, too. We fed them once a day in the winter. Otherwise, they fended for themselves. One of them came in the house one cold winter night when she was about 16 years old. She never went back out and lived to be 22 as a lap cat.