Feral cats are not native to any wild environment, including urban environments. They were bred for centuries to be indoors. Outdoors they get terrible diseases and they hunt anything else that moves. Cats don't belong outdoors, and feeding them only makes more cats that suffer and kill other animals that belong in the ecosystem.
The previous poster was correct to point out that many wild birds are killed each year by cats - but they got the number wrong. The American Bird Conservancy estimates that 2.4 billion wild birds are killed by cats in the USA every year - not millions. Estimates by scientists range as high as 4 billion birds annually in the USA. And estimates for the number of mammals killed by cats in the USA range from 6.3 to 22.3 billion annually. Spay-neuter-adoption/release is the only humane solution to the tens of millions of feral housecats now loosed upon the country. Feeding them outdoors only makes the problems worse and causes more suffering.