I would have said dogs--my neighbor had a pit bull that would bolt through their electric dog fence and jump the 7-foot chicken fence to get at the birds. Luckily the dog has passed on. I have limited most problems by fencing in the birds in a 100 by 60-yd pen and making sure they are locked in at night or when we are away from home. That being said, last spring a weasel managed to find an opening under the chicken house door that allowed him access to the interior. He would kill one or two birds every couple of days and, unless you examined them closely, the cause of death wasn't evident. By the time I woke up to the fact the birds were being killed and not just dying (these were mostly 5- 6 year old hens) I closed up the entry hole then live-trapped and eliminated the culprit--he's in a far better place.
When I kept ducks--5 years ago--I lost many of them to coons. They would sneak up on them when they were asleep at night, even when they were on the pond.