Do you believe when you hear on the internet that feral cats are no or little threat to our birds and fauna? The very few ecologist, biologists, or veterinarians I've seen use that line are usually closely associated with or manage their own trap, neuter, return, and other cat organizations.
I will refer you to "The Internet Center for Wildlife Damage Management" brought to us by Cornell University, Clemson University, University of Nebraska -Lincoln, and Utah State University. Please pay special attention where it is said:
* Kills young of songbirds, game-birds, rabbits and other wildlife; but rarely anything larger than ducks or pheasants. Scientists now list invasive species (including house cats) as the second most serious threat to bird populations around the world (Source: Connecticut Wildlife Mar/Apr 2007 p. 18).
* Messy feeding behavior. Portions of their prey are often strewn over several square yards in open areas. The meaty portions of large birds are consumed entirely, leaving loose skin with feathers attached. Small birds are generally consumed and only the wings and scattered feathers remain. Cats usually leave tooth marks on every exposed bone of their prey.
* Disease hazard- see below.
* Use gardens as litter boxes.