Ted Williams wrote in his Incite column in Sept./Oct. 2009 in Audubon magazine about the problem of feral cats preying on wild birds.
Feral cats are not very common where we live in Vermont probably because the cats would have too many predators that could prey on them in this mostly rural, woodland state. Clearly any feral cat populations will add to the already significant problem of cats preying on wild birds. This article is good background reading.