Strange as it may seem, your cat is actually trying to please you with its hunting skills, literally when it leaves its prey on you patio or at you front or back door,what it is saying to you in its own way is "look what I got for you, I brought this bird to you!" You may notice that your cat never eats the bird but just plays with it and even tortures it,so its not like the cat needs it for food. Unfortunately this is a very deep inctinct and short of keeping your cat inside,there is really not a lot you can do. The bell around the neck is a very good suggestion,but try as you might you will not stop it entirely. My personal view is if you do try to stop your cat from being a cat,you will make it into a very unhappy cat! Its a natural act for yout cat.