Cats- the number one predator of birds.  The only thing that you can do to keep your cat from killing birds is to NEVER let it leave the inside of your house.  Stray cats are an even bigger problem, because you don't know where they are most of the time.  You can keep stray cats out by putting up a tall fence with very small holes.  If you have an outdoor cat, it will probably kill your birds. It's not a cat's fault, though, as many people may think.  It's their instinct to hunt and kill birds and small mammals.  You would have to go back (or train) generations to get rid of this instinct, and even then it probably won't work.  The best you can do is keep your cat inside.