I just googled black birds native to GA marshes & came up with "Cormorant." Kinda looks like that to me. As for the headless part, looks like a racoon could've gotten it.
They're notorious for that. There is a protein in the brain tissue that they like/need, so that's what they go for.
I strongly suggest putting out some traps. We use marshmallows in ours. We catch and release...into my the light. In my opinion you do NOT want those critters near your flock. And if that's what it was, it's going to be back.
Your coop secure?