Very sorry for your loss.I can't find the heads anywhere I can't see any holes or places in the wire were an animal could have gotten in what could have got them all
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Raccoon usually take A head off reaching through the wire. That many dead birds all with heads missing sounds like something in the weasel/mink family and they can fit through holes as small as 1 inch. Covering any holes with 1/2 inch hardware cloth is recommended. The weasel type animals go on killing sprees and get anything that moves. Were the bodies stacked in a certain way or anything like that?
The only others I know that go full on spree's would be domestic dog type, (doubt fox or coyote do that but maybe), Doesn't sound like that's a possibility though if they were locked up. Photo of your coop design might be helpful.
Probably too late.. When a tragedy like this happens, we TRY to have the mindfulness to process the meat for the freezer instead of wasting them. Took a LONG time to reach that place though.
Hang in there!