That makes more sense.UPDATE TO EVERYONE. THAT IS ACTUALLY UNDER THE CHICKEN THAT IS IN THE PICTURE AND NOT ITS BACK LIKE I THOUGHT. I THINK I JUST DIDNT WANT TO LOOK AT IT TOO LONG. Plus no one here same to notice it was under the chicken either, so I dont feel too bad.
The owner said her coop is not crowded. The only thing she could think of is that she recently change to wood chips for bedding. I told her to just monitor the chicks more closely.
I actually had asked the question about the location of "injury" in my post too, but then your title says "Back" so I assumed I was just looking at the photo incorrectly so I deleted that part LOL The vent looked upside down to be viewing the back, but then again...
So this is the underside. Bird has a lot of feces caked on it's underside. Was this bird mobile? Meat birds don't move around a lot, so it's legs may have not been working well.
Did she happen to cut the bird open and see what the liver and internal organs looked like?
Where are it's legs, did she remove the leg/thighs before the photo was taken?
Just by looking at a photo it's just a guess, but I would say an immobile broiler. The redness looks like blood pooled under the skin to me. The others took advantage and cannibalized a handicapped bird.
As for why the bird was not well - organ failure? bird too heavy to move? disease?
It's all a guess unless she has more info or photos of what the organs looked like and someone can help i.d. an issue.