It's really hard for me to see in the picture, so I could be wrong, but flystrike is when flies lay their eggs on a chicken, and the eggs hatch and the resulting maggots start tunneling into the chicken. If you look closely, and there are maggots, that is what you're dealing with. There are several threads on treating it here on BYC, if you search flystrike.What is flystrike and how can you treat it?