If they are already around i wouldn't bother feeding or spending money at the vet on one. Feral cats get feline AIDS and leukemia and pass it around to each other. Vets can't do much for those diseases except test for them so feral cats usually don't live long. And if you have a pet outdoor cat...
We have lots of strays in my neighborhood and also outdoor cats. Our cat is outdoor as well and since cats have territories she defends her territory (our yard and several neighbors') from other cats. A couple still wander over but they have never bothered my birds even when they're in the yard