tough one. my guess would be raccoon.
bite marks don't match a bird of prey.
dog would have that thing coated in slobber, and wouldn't have tried eating it (unless wild, but it would have done a better job than what is shown)
large cat possibly, but seems like any large animal would have eaten more, unless disturbed in the middle of eating (had she just been killed, or had she been dead a while?)
raccoon would have the dexterity and ability to sneak into your coop if it was unlatched, and they would match the actions of biting the head and neck and ripping out the throat. but, it seems like it would have eaten more than what is shown, unless if disturbed
a weasel of some sort could sneak in, but from what everyone tells me and from what i've read, it would have killed more than just one, it would have went on a killing spree.
my guess would be raccoon
edited to add: looking at the pic again, it would appear whatever it was, was going for the guts. again, sounds like a coon