Are your birds fully grown? Most hawks or owls (at least in my area) won't carry a grown chicken away. Usually will kill and eat on site. I'd lean towards a fox or coyote, and one that's very talented at making clean kills. A kill site that looks like a chicken exploded--feathers everywhere, but no bird--is a good indicator of a fox. And they can be pretty, for lack of a better term, ballsy. It's not uncommon for them to hunt in broad daylight. However, it is interesting that there are only a few feathers left. In any event, it's something big enough to carry the bird away with it!