6chickens in St. Charles :
I am a nurse of humans! So I dont know if chicken flesh heals like human flesh, but I hope some of this information may help you relax about that wound:
Open wounds heal by "intention", from the inside up, and the first step you'll see is a layer or two of collagen. It looks like the sandy yellow glops over the surface of the wound. Seeing a deep golden,yolky colored tan/yellow layer is a good thing. With good nutrition, the next layer is granulation, bright red new tissue, also looks sandy, then a pearly pink top layer. That will look like a scar forever. The trick is to keep the whole thing from drying out while it's being made. Also don't continue to break it down with hydrogen peroxide or betadine which will both kill the new tissue. But we often paint betadine on the skin surrounding the open wound to help prevent normal skin bacteria from moving into the wound.
Maggots clean a wound and prevent infection. One fly can make lots of maggots. A wound with maggots usually has no odor, because the maggots eat the dead flesh away, preventing bacterial and fungal infections. I've seen many maggotty wounds on people, but I've never seen maggots in healthy, live tissue. It may happen, but I've never seen it.
The biggest concern for a wound that big is the total percent of body mass it takes, which could cause rapid dehydration and death. Skin (and feathers) keep our moisture in, and temperature controlled. Now her whole back is exposed, and her heat and moisture will just escape. She could be dehydrated in a few days and just die. So if she were a person, we'd put damp sterile saline gauze over the wound and change it 3-4 times a day to keep it from drying out, being careful not to peel away any new tissue/collagen she's grown in. And we'd keep the room temperature warm so she'd spend less energy on body temp and more energy in wound healing.
That area of pale, whitish skin towards the chicken's tail looks dead. It will probably have an area underneath it we'd call "tunnelling", where her body may create a dry pocket which could get chronically infected, so a vet might be a really good idea to clean up that wound, clip away parts that might prevent the whole thing from closing "by intention".
I wish you and her the best luck. I wonder if chickens heal the same way people do? Chickens seem to be amazing and resilient, they recover from a lot. I hope yours does too.
I agree with the Peroxide statement. I heard that it can also kill away the new skin cells. It is good to use a Betadine / H2o solution to kill Bacteria. It is a good thing to have on hand! I actually used it when My Greyhound cut her leg.