Maybe the question is how attached are you to the chickens? If these are beloved pets that you will be devasted if something happened to then I think the suggestion of a covered run inside the 1/4 acre is a fine one. You can let them loose in their paddock when you're around to keep an eye out and when you're not available, lock them up in a their covered run. If these are "just chickens" which you don't mind replacing on occassion, turn 'em loose. Whatever you choose, it will probably be an ongoing project anyway. There are some threads which deal with hawks and I saw suggested that you can run fishing line across large areas. Apparantly the hawks can see the glint and won't try to fly down. Inexpensive save for the poles you'd need to put up to support it. Do a search and you'll find some more ideas for large area.