Cats on the prowl and hawks hunting can definitely be a problem for you when you set out bird feeders. The first thing to remember is that birds are smart and recognize threats when they see them. So, the best thing you can do is give the birds a chance to see anything that would threaten them.
If the cat problem persists you may have to consider a fence. We all know that cats can go over or around fences, but the purpose is to give the birds time to see the cat. Try using a green vinyl coated fence with a 2"x 3" mesh. Birds as large as Thrashers and Robins can fly through this mesh. Put a section of the fencing in the path the cat attacks from. Try to force the cat into a path that can clearly be seen by the birds.
If you have a hawk, try tying surveyor's tape across the path the hawk prefers. All of the Accipiter family of hawks like to fly low through the trees. The tape forces them into a new path giving the birds time recognize the threat. Surveyor's tape can be found in most hardware stores. It comes in brightly colored vinyl about an inch wide.
I have had people report other ways of solving the problem. Shaking cans with coins while charging out your door or spraying the cat with water. These techniques may work at the moment and are based on the fact that you can see the cat before it is too late. I can't say how well they would work on a long term basis. It is best to give the birds a chance to see the danger and protect themselves. You may want to learn more about attracting birds with these articles.