It's hard to say. The wound could have been caused by about anything. What did your kill sites look like? Generally with a bird of prey, they sit on the carcass and pull out a bunch of feathers, so there is kind of a messy pile, but all the feathers are dry. Hawks don't slobber.
I can't really tell, but are the feathers on the duck kind of wet? That would indicate a dog... they grab with their mouth and leave slobber.
In any event if you can, leave your flock locked up in runs/pens so the predators can't get them. Sounds like *something* is after them.