I had gone away on vacation to see my grandparents for a couple of days. I can not stress the importance of having (and keeping) a good chicken sitter because you never know what is going to happen while you are away. A knowledgeable sitter can help treat your birds if the need arises.
As murphy's law, entropy, ect. suggests that of course while I was away, my chickens would sustain their first predator attack. I'm going with said predator was a stray cat. I live in an area that has few predators due to a large buffer busy roads and lots of activity from humans consistently. We do get the occasional fox or raccoon but my sitter helped me to figure out that it was unlikely that the culprit was either of these animals. Specifically, the wound on my bird and the feather loss on the others was on the backsides of each of the birds. From my understandings, racoons will rip out the crop and front sides of the chickens when preying on them (I can't find anything confirming this online though, Just what I have been told). My feelings about this is that a stray cat pounced.
CJ came over to check on my chickens to find many feathers around the yard. Kasima was huddled behind Wuba who was protecting her. She had a very deep (to the bone) wound on her wing. Several of the other birds had tail feathers ripped out. CJ's first text message to me was not to panic. Which is sort of funny, but actually very important. If you're birds get attacked don't panic. Do your best to stay calm and evaluate the situation. The first thingI can say to do in almost every situation is; if the bird is still alive get it into a safe separated area from the other, hopefully, non wounded birds. This is because your other chickens will instinctual peck at the wound causing much more damage to your wounded one. This is all the 'advice' that I will confidently give. Every situation is different and it is important you use your best judgement for what needs to happen.
In my case, CJ had separated Kasima with a dog cage placed inside the coop by the time I got home. The cage was lined with bedding and had dishes of both food and water in the cage itself. (It's a good idea to have a set of the mini feeders and waters like the ones used for chicks because it is not uncommon that from time to time you will have to separate a bird. It is way less stress on you and the bird to try and come up with 1) a way to separate the bird and 2) a way to get the bird food and water on the fly).
We made sure that Kasima was both eating and drinking, which she was. This was a great sign. CJ suggested that I use bactracin ointment to treat the wound. I applied the ointment twice daily for the first couple of days. Kasima did have some 'watery stool' but I will attribute this to nerves. It is important to keep the coop or area where the wounded bird is dry and clean. You don't want any flies or more bacteria than necessary getting into the wound. So with a clean area and the bactracin ointment being applied twice daily, Kasima appeared to be continuing to do pretty well. Around day three, she seemed to have formed a pretty nice scab. I continued with the ointment at least once per day for the next week. Something that I started doing around day 2 or 3 is that I used the wound wash saline to clean the wound before applying the ointment. This seemed to clean the wound out pretty well (I use this stuff to clean all my piercings and tattoos as well).
While she was healing I provided Kasima with her regular feed, protein in the form of DRIED meal worms, the occasional fruit and vegetable treat, and a vitamin and mineral supplement added to the water. Be sure to clean out the water often, I suggest daily. CJ and I briefly talked about antibiotics sold at local feed stores but ultimately decided against it due to Kasima seeming to be healing well. I also made sure to keep track of the temperature of the coop and place Kasima in a place within the coop that got fresh air and some filtered sun so that she wasn't too hot or in the dark all the time. I'm sure if you wanted to add a toy your chicken would appreciate it as it must be very boring being cooped up like that. Kasima's buddy, Wuba, kept her very good company during this time by talking to her almost constantly from outside of the cage.
After a little over a week, I let Kasima out of her cage on a test run to see how the other chickens would react to her. It helped that I kept her in the coop while she was healing so that they didn't forget she was a part of the flock. The chickens did not pick at or on her during the day. So I decided to let her free range with the flock during the daytime hours. Because my chickens are free range within my yard, it is easier for Kasima to get away from any bossy chickens that might hurt her wound. For the next week to two weeks, I would put her in the dog cage at night to ensure that when everyone woke up in the morning she wouldn't be picked on.
It has been over a month now and Kasima appears to be doing well! Some of the feathers on the less wounded areas have grown back almost completely. The more wounded area does not have any signs of feathers and I am unsure if it ever will. Strangely enough, the attack appears to have brought the flock closer. Since my chickens are weirdos who don't roost, they now all sleep in one giant pile instead of in separate piles on opposite ends of the coop.
In conclusion, it is important to be prepared. Have a plan for separating out your birds in case of emergency. Keep something like a cage and small feeders and waters around for quick use. Even though you should always do your best to predator proof your coop, things happen and it's essential to be prepared for as much as possible to treat your birds appropriately. Also, always consult a vet if you are unsure about your birds health. You can find vets that will see chickens in your local area by searching in google "chicken vets [your city]". You could also try here and here which have a listing of vets by state. I found a great vet who will see chickens this way when one of my young birds was displaying splayed legs (saddest ever) but she was very helpful and informative.
As always, best of luck to you guys and I hope this helps.