posted 3 years ago
This happens - chickens can be quite nasty:
1. The most common reason I see is that the bird in question in fact is not well and since when the flock "tries to chase the sick chicken away" she has no "away" to escape to, she gets injured. Some illnesses are quite hard to spot, but check her for lice/mites and particularly for wasting disease and see how she does separated.
2. Another common reason is that the bird is getting a little prolapsed when she lays and that's also an open invitation to pecking. Extra magnesium may help that, but step one is to even see if she's still laying.
3. Sometimes, there's a bully in the crowd and others follow that example. All you can do is watch the flock and see if 90% of the time, it's the same bird starting the trouble. Have a super soaker with you, and shoot the bully in question with a decent spray of water and you might teach her to stop!
4. This is a good example of why it's good to have extra infrastructure. We have ducks and we regularly end up with a Miss Dickens or 3 - a "chicken" that lives with ducks" is a Dicken! Our most recent Dicken was from #2 above - she's healed up, and we think she's laying once every 3-4 days, but those darn chickens don't engrave their eggs with their name!
5. Reintroductions are hard, and I agree with all the suggestions Matt makes and the need to have distractions for the whole flock when doing so. That said, I'd also be prepared to yank her back out and try again the next night, and again, for several tries. If a chicken takes on too much of the "cower in fear" role, you will need to find another alternative longer term, but cross that road when you get to it, and during the introductory trials, try to see if there's a bully instigator. This is important because you may find that with the current target out in sick-bay, the bully will target someone else. The difference between "the top of the pecking order" and a "bully" is subtle but I've seen it, and the last thing I need to do then is keep removing injured chickens!
6. Is having a "sub-flock" something you can do? There are times when that can be useful to have, particularly if you occasionally have to refresh your flock from off-site.