This isn’t the first injured squirrel I’ve had spend a lot of time in the feeder. Sometimes, I think they managed to escape a hawk or some other larger predator. But squirrels fight each other too as do most other small animals. You’d think that given how fragile life is for them that they would stop attacking each other, but that’s not the way it works.
Rabbits fight and can (and sometimes do) kill each other. Squirrels don’t seem to be as lethal as rabbits, but they pull each other’s fur out. Especially tails. This guy looks like he lost his mulligan in a bar room brawl. Not badly hurt, but wounded.
Why do they fight? They fight over food. The boys fight over the girls. They fight for dominance. Sex causes a lot of trouble among all species that have sexes. Probably not so much with worms who don’t seem to be gendered.