I rescued a baby field mouse from Cassie, about 8 yrs ago. I was going to set it free after it had recovered, but it was doing well in captivity (as it was very young) and I got all soppy and didn't want to set it free, just to have it caught again. Anyway, to cut a long story short, we kept her and she became very tame. we fed her on rabbit food, plus berries and nuts in the autumn and she lived to be 6 years old! She moved with us to Wales (in her little box in the back of the car with the two cats we had then) and had a big terrarium, with loads of space in our study. I must admit, I felt a bit sorry for her, being on her own, but she seemed to enjoy our company and used to come out and run around. When she finally died, I buried her in the garden, under a young conifer, which has now grown to about 3ft. She never had a name, just "mouse" and she was very sweet.