What preys on hawk?

a snake.

Ground dwelling animals such as coyotes etc. prey on the young and eggs of hawks if they can reach the nest. Also one type of hawk will readily kill another type: for example, a goshawk would kill a Harris hawk if it had the chance - for that reason when falconers attend a meet they only fly one bird at a time! There are some exceptions to this such as Harris hawks and caracaras, which often hunt co-operatively and share the kill.