I have never seen my hens actually catch a mouse, but when the farm cats come with a dead mouse, quite often the hens will run at the cat and grab the mouse and eat it.
I have seen the hens, quite a few times, jump in the grass next to the stream, and come out with frogs, kill them and then eat it.
I suppose to the hen, its food, but it seems strange they will eat things like mice and frogs, to me!! Not sure of the health implications, but doubt my egg customers would be happy if they knew!!! There again, the hens are still here, so cannot have damaged them much, eating strange things.