When I were a yoof I worked on a chicken farm, being responsible for feeding and watering the birds. The farmer had sold all his land to a building co (they only ever got planning permission for 16 of the 200+ acres, thankfully) and rented one field where he kept hens, a few turkeys, a cow and some pigs.
His hens were on "deep litter" - they were free to roam around inside sheds - and the houses were partitioned. One of these partitions had been patched with an older paper chicken food sack.
I can't remember why, but we had to remove this sack for some reason - and it was full of mice. Dozens of them came pouring out, and the hens had a fine old time grabbing them and devouring them avidly.
I've also known hens attack one another. They definitely establish a pecking order, and sometimes the poor bird at the bottom gets pecked to death by the others. I can remember helping my dad, who also kept hens (he bought a couple of the sheds from the farmer's widow) covering open wounds with Stockholm tar.