I'm afraid I don't have an answer for you as my cat does the same thing. We recently moved house from a town property to the countryside and my cat, Smudge, is in heaven. He has never even seen this much wildlife and is trying to catch everything he sees. We have had mice (alive and dead), birds and rabbits all brought home as a present of love for us! We have tried all kinds of collars with bells on but none work, he manages to get out of them all and leave them chewed somewhere in the garden! I don't have a cat flap as it is easier for me to let him out and then shut the door so when he does bring things back, at least they can't be brought in the house. He has learnt to come to the patio windows when he is ready to come back in and I think this is the most trained he'll ever be but it's ok, as horrible as it is to see, it's nature and I love him no matter!