Raccoons:
Growing up, we had quail that we raised in the back yard. We lost around half of them to raccoons who were able to reach into the run and tear the birds apart, but the holes were too small for them to pull any part of the birds out. Heads, wings, bodies and legs well separated and left at the bottom of the run. Not sure of the exact number of birds, as it was years ago, but I'll bet at least 10 were killed before the raccoon gave up. It was horrifying to see the damage!
My brother lost 6 chickens (4 red sex link, 1 black sex link and 1 bantam) to a raccoon who reached into the run and pulled the birds apart. He was very attached to the chickens- they were quite friendly and tame- and was very sad when he went out to the yard and found their headless bodies in the run.
Hawks:
My brother also lost free-ranging chickens to hawks.
So far nothing has attacked my 4 yet (knock on wood), but the predator I most worry about is the raccoon. When I let the hens out to free-range, I stay in the yard to discourage the birds of prey. My yard is securely fenced to keep out dogs (although rabbits have dug under to get to my garden, so I am not counting on that to keep out foxes or coyotes). I have made the "coop" (my shed) and their run as secure as possible (in my estimation) so I am hoping they will be safe. I know that raccoons will try anything and everything to get a taste of chicken, and may be clever and bold enough to find a way in.