There's a good new comprehensive article on feral cats now available.
I've seen their effects on birds and small mammals. I bought a rural place that was once a dumping ground for unwanted cats. I work in my garden, disgusted by signs of their presence. I've stopped feeding the birds in the back yard because the feeder became a killing-place. I may remove the bird bath; it too has become a hunting blind for the "cat-children" of "neighbors."
I have information on my web site but it probably needs new numbers from studies to become more convincing: there is no place for free-ranging house cats in urban and border areas. House cats need to be kept in houses where people love their cats. Cats are very effective killers of all small reptiles, birds, and mammals. Most are food and energy subsidized by people and cannot be equally compared to "wild" populations. Urban ecologists, if ever they are to include being "managers," need to tackle this problem