What i have found to be affective against large birds of prey is to use a large form of bird myself, my geese usually warn away most larger birds but then again buzzards actually nest in my barn so i dont know if they even mind the buzzards.
Point being that usualy if my geese are around hawks dont come near my ducks.
Also haveing ducks that are expierenced with these kinds of things help, for instance my mallards ,when they see a hawk, quack as loud as they can and fly everywhere its like a blizzard of mallards, muscovys, and giant geese.