We used to have up to 5 cats at a time ( now all gone to the great littertray in the sky, alas ) and we have 2/3 of an acre garden in the middle of a rural/agricultural environment. In all the years we had them they slaughtered a handful of garden birds ( and were severely reprimanded ) and hundreds and hundreds of mice, voles, rabbits, moles and rats, which were, of course, much easier to 'pick off' than fluttering birds. Since the cats have gone, I have had rats living and breeding in the sheds for the first time, mice in the house, and voles and moles in the garden. Don't disparage the domestic pusscat too much, they earn their keep. All self-respecting cats would much rather kill and eat a rodent than ever they would a feather-filled garden bird.