predator?

I woke up to all my chicks [except one week old chick who I found alive squished in a crevasse] dead in their outdoor elevated brooder a bin with it's lid tightly on which is solid plastic with 1/4" hardware cloth. Everything was in place and it doesn't look like anything was there or messed...

I have 2 duck coops, they are both pretty big, my one coop had 2 Indian runners and 2 chickens and the other one had 2 mallards and 2 pekins.. recently my 1 runner died and my 1 mallard died. No missing feathers no blood etc! Separate cages! What do you think caused their death? They are eating...