From the description, extraterrestrials and satanic rituals are more likely and bird predation unlikely.
Satanism is my guess.
Vultures would likely be the first to find the carcasses. No way they cut through the skin on the back. With a fresh carcass, they’re limited to dragging organs out through an orifice. Makes me wonder about the reproductive system being gone. I saw a large cougar that a trapper had taken. The abdomen was deflated. I asked him how much it weighed, and he said “ Probably about 150 pounds, before the buzzards dragged its guts out it’s ass.” Didn’t see a mark on it.