My Rooster Has A Golf Ball Sized Swelling On His Neck

First sentence says it is a pet. One does not cull pets.

I don't think Mareks tumors show up externally like that, although I do know there are a couple of uncommon types of Mareks.

A vet visit would be a good thing. It may be an abcess, and it is sometimes difficult to treat them without lancing and draining to remove infection. THere does appear to be something going on with the feathers, although if the swelling is painful or itchy, the bird himself may be biting at it to try to relieve the pain/itch. I don't know enough about ingrown feathers to harbor anything more than a guess, but it could well be that.