I had this happen to me to. I dont have any hawks around where I live that I know of, but I had a large amount of blue jays, I ahve came to the conclusion that the Jays had something to do with the disappearances, I havent had a problem since the jays "left" but now I have the occasional crow so far they havent taken or hurt any of the the chickens. I have heard plenty of stories of crows killing chicks and chickens though.