Please i need advice and an answer about mallard wild ducks.?

Everyday about 10 ducks fly in my yard and lately i have 4 ducks with broken or badly damaged legs and feet ' where they fall over if sitting' and who is doing this to them.I know the males are rough when they mate' but this is torture not love.Could the crows be doing it to them?What do you think happened' and what can i do to help.They won't let me get 6 feet from them.Thanks so much.