The only chicken kill I have had with possum, which I think was possum. They start at the anus and eat the insides out. Most possum I get is in the winter because they do not hibernate. The possum I usually get in winter eat the chickens food. The one year I had a larger possum had a bird with...
Check the dead roosters for other biting like in the back of neck. Could be a one off coon. Mink, weasel family lap the blood and or try to drag the animal away if they can.
Id go with a weasel or mink, possibly a rat. Check the other chickens for any bites around the back of the neck.
Whatever it is will be back, Set up a game camera.
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