Small mammals is kind of the point. There are mice in the garden that try to get into the house; the cat's job is to kill as many as possible. Regarding birds,

Small mammals is kind of the point. There are mice in the garden that try to get into the house; the cat's job is to kill as many as possible. Regarding birds, I'm not sure. It can't catch magpies and other crows. We don't see or hear other birds (we live outside Paris and don't have that many birds). I don't think it would make a lot of difference to keep our cat indoors since all other cats in the neighborhood also live outside -- except upsetting and depressing ours.