Yes, I agree with Henry. We lost a lot of baby ducklings to rats the same way. The survivors had injuries on their thighs. First we were blaming the cat who suddenly was always in with them, then we owed the cat an apology. It wasn't the ducks he was after but the rats.
CAT!! When I lost 23 in one day the cat had done the same thing. Ripped one leg off each and every chick. And cat had got inside pen, didn't rip them off from the outside.
We are thinking cat right now, i have a large male that was caught last night trying to get at them threw the hardware wire on the side last night. Im not sure if he will be back around, DH shot at him with the pistol.......he doesnt like cats all that much......
I would not put defenseless chicks in a brooder outside at night without a mother to protect them. Mine that are out of the brooder are not even outside at night yet. I want them to be full size before I put them outside in a coop.