Any ideas as to what happened?

[Ayla_noemi]

Chickens will do that to a flock mate sometimes. All it takes is a bit if blood, like a broken pin feather, a minor boo boo, or even a prolapse vent. Chickens can be quiet carnivorous...and will peck/devour another flock mate like that. Once There are a few pecks that create an open wound..the others will all go into a feeding freenz. Did u look at the others for signs that perhaps they were the culprit?

I did not notice any blood on anyone else but to be honest I don’t know what else to look for.

 

[lazy gardener]

No signs of blood anywear besides all around her backside. I did not see more feathers than normal like when a predator attacks. My pen is 12x 16 and there are about 15 birds in there but most are male which is why I though over breeding was the cause. The fencing is chicken wire and the doors are treated wood and chicken wire with correlated metal for roofing and wooden nesting boxes. The pen is closed at night since my dogs sleep inside the house. They free range during the day. I have separated the girls and boys for now in different pens. Will the boys fight each other if they see the girls in the other pen or will the relax? This has never happened since our ratios in the other pens are 1 Roo to 6-9 females they are all the same dimension.

Chicken wire is not predator proof. Is the top of your run covered? While I don't think the damage was caused by a weasel, one could easily squeeze through the chicken wire. Also, a coon could shred the wire or simply reach through the wire and grab a bird, pull her to the wire and munch away through the wire. Any of the coon size or larger predators could easily shred the chicken wire.

When you say your pen is 12 x 16, are you talking about the coop which is the closed in area where the birds roost and lay eggs (this is typically called the coop) or are you talking about the more open area which is typically open on all sides, closed in with fencing, and may or may not be covered over the top (this is typically called the run)? Or does the 12 x 16 area consist of combined coop/run?

15 birds need 60 s.f. of coop and 150 s.f. of run space. This is if there are no integration issues, and if there are not multiple age groups or more than one roo.

Separating all the cockerels/roos away from the pullets/hens is a good first step. I keep my cockerels in an adjacent grow out coop where they can see the rest of the flock, and the rest of the flock visits them when ever they are allowed out to free range.

 

[rebrascora]

I agree with what the others have said. This is not mating damage as there is no penetration involved in chicken mating. Mating damage almost always involves injury to the top side of the chicken like scalping or feather loss and sores on the shoulders or back or gouges to their flanks beneath their wings.
It is difficult to assess whether it may be a predator without seeing your set up and where she was found but my gut feeling is that she was probably cannibalised due to an illness. Too many cockerels will really stress a flock and when they are stressed they are prone to outbreaks of diseases like Marek's which can suddenly make a bird very vulnerable and then the whole flock will turn on them. As others have suggested it could also have been a prolapse. Removing the cockerels was definitely a good first move because their raging hormones at this time of year cause them to make the pullets' lives a misery and once there is more than one cockerel with no mature birds to keep them in check they will repeatedly and mercilessly mate whichever one of the pullets can get hold of.... usually the weakest/slowest one. Taking the cockerels out will also give the remaining girls more room which should also offset any tension.
The second step would probably be improving your coop/pen security from predators. Chicken wire will keep chickens in but rarely keep predators out.

 

[Ayla_noemi]

Chicken wire is not predator proof. Is the top of your run covered? While I don't think the damage was caused by a weasel, one could easily squeeze through the chicken wire. Also, a coon could shred the wire or simply reach through the wire and grab a bird, pull her to the wire and munch away through the wire. Any of the coon size or larger predators could easily shred the chicken wire.

When you say your pen is 12 x 16, are you talking about the coop which is the closed in area where the birds roost and lay eggs (this is typically called the coop) or are you talking about the more open area which is typically open on all sides, closed in with fencing, and may or may not be covered over the top (this is typically called the run)? Or does the 12 x 16 area consist of combined coop/run?

15 birds need 60 s.f. of coop and 150 s.f. of run space. This is if there are no integration issues, and if there are not multiple age groups or more than one roo.

Separating all the cockerels/roos away from the pullets/hens is a good first step. I keep my cockerels in an adjacent grow out coop where they can see the rest of the flock, and the rest of the flock visits them when ever they are allowed out to free range.

It is completely closed off so I guess it’s a coop.