Adults killing young birds
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The nature of the pecking order is that bigger birds peck at smaller birds. When raised in a flock environment, given enough space, adults generally leave young birds alone. When introducing younger/smaller birds to an established flock they immediately become targets. In the process of pecking, the adults may kill and consume part of the young birds. If something is eating whole carcasses, there is another predator in the mix. We need more information to give a better idea of what is going on.
We also have 35 or so 9 week old chicks. A bunch of different varieties. 9 are dark Cornish cockerels,10 are dark Cornish hens and the rest are BO, leghorns, rocks.....
The little ones are a 6x6 at night an free range during the day. The big ones are in a portable coop and free range all day as well.
Two little ones turned up dead in the middle of the day in the small portable pen. Not much is gone so I doubt a predator, just wanted to check.
Not impossible that the roosters might be trying to breed the smaller birds and inadvertantly killing them. Also snakes will someime kill birds that are too large to swallow - in this case the heads and necks of the bird are generally slimy. Good luck at resolving this.