When you say "not on the ground" do you mean you're not feeding them on the ground, or the cat isn't killing them on the ground ??
I'm assuming you're feeding them in feeders, but the cat is killing them on the ground.
As already mentioned "I do not consider that it is every householder's responsibility to make his (her) garden "cat-proof" ..... however, sometimes this is what you must do.
I don't know the set up of your garden, but have you considered them strips that you can put along the top of your fence with little spikes on them - virtually impossible for a cat to climb on or walk along ??
Have you considered getting a dog, or even borrowing one for a week or 2. We've 2 Yorkies that can come and go through the cat flap as they please ..... absolutely NO cats come in our garden !!