There was an article in (I vaguely recall) "Sports Afield", some ten years back. It referenced a study done by the Wisconsin bug'n'bunny agency, using a bunch of wildlife biology grad students.
Conclusions: One feral cat will kill as many as 100 songbirds a year. Estimate: As many as a million feral cats in the state of Wisconsin.
You do the math.
Also, it was stated that at one time the state REQUIRED hunters to shoot any feral cat that was seen. They were smarter, back then...
NOTE: We're not talking about Persian Fluffy in town, in the house, in your lap. "Feral". Look it up.
Art