We know for a fact that a fox is what got all my daughter's birds as it kept coming back to check on the coop long after it had taken all the birds she had. It was common to spot it at all hours of the daytime. A small tuft of feathers was the only clue it left.......aside from that, the birds had simply vanished. My guess is the small tuft of feathers is where the fox stops to kill the bird it has captured, and once it stops struggling, the fox then takes it off somewhere to it's den, which may be some distance away.