Other hawks, eagles, owls can prey on them. The Goshawk is well known for removing any competitors from it's territory, even larger species. Large owls sometimes take hawks that are roosting at night or even when they are incubating eggs. Any type of cat, or dog, or marten, indeed any carnivorous mammal.
The hawk because energy is lost at each stage of the food chain, and the hawk is the last stage. This means it doesn't get all the energy transferred to it.