I find leaving feral cats to defend for themselves a cruel decision. I live in the Boston mountains of the Ozarks in Northwest Arkansas, and am surrounded by thousands of wooded acres. We had a feral cat living on our property when we moved out here and watched him closely during our first Spring. We named him "Dorsey" and wondered just what he would do when Winter rolled around. Covered in ticks all Summer, his weight dropped significantly and we began to feel sorry for the little guy; clearly he was suffering. My wife suggested we feed him to put some meat on his bones to help him survive the Winter, so reluctantly I agreed. By Fall, he was a strapping male brute of a cat and began to harass our own cat when she was outside. Concerned and thinking he had plenty of fat stored to survive the Winter, we stopped feeding him as we moved into Winter. We didn't see him thereafter for a long time and then when the following Spring rolled around he reappeared shriveled and sickly looking. Once again we found ourselves feeling sorry for him but this time we felt it best we end his suffering. I dispatched Dorsey with a Ruger 10/22 and it wasn't easy considering we'd kind of adopted him there for a while. Long story short, I should have done it when we first crossed paths; local animal shelters are already brimming with the unwanted "pets" that are the byproducts of human irresponsibility. Most will never see adoption and will be euthanized. Give the ones that are already sheltered a chance by eliminating the ferals you find; you'll likely be doing them a favor.
