Yep, it's a Cooper's hawk, as far as I know. Our area is thick with them.
We haven't lost a chicken since we got the pups back in December. They sleep right outside the coop door every night, and when it rains they get under the coop by the door instead of in their dog house, which is attached to the back of the coop.
I can't WAIT to see how well they guard when they're older and more mature, given what they're doing now.