Should also mention... Periodic migrations of sorts, of parasites, can occur, where you can have such bizarre yet not uncommon situations as having a tidal wave of sand fleas many miles from the coast, or leaf mulch fleas from forests, or whatever, that just march across your property and maul everything in their path. Sometimes it's a natural but erratic population explosion, sometimes it means something in the environment has been altered for the worst. Sometimes it just means temperatures rose or dropped at a different timing than usual, so, for example, midges hatched sooner than usual while the bird populations that normally eat them had not yet arrived or weren't yet at sufficient population densities to control the amounts hatching.
Some population check or control could have temporarily failed, and you could be dealing with an overpopulation of some resident species that bites or drinks blood, which your hens could be showing the effects of; it's also possible that your rooster overpopulation, possibly combined with the hens going into moult, has caused or contributed to this problem. Some males are extremely nasty to hens if they feel desperate, i.e. when being regularly hounded by other males. No excuses in my mind, I cull them too.
Best wishes.
Some population check or control could have temporarily failed, and you could be dealing with an overpopulation of some resident species that bites or drinks blood, which your hens could be showing the effects of; it's also possible that your rooster overpopulation, possibly combined with the hens going into moult, has caused or contributed to this problem. Some males are extremely nasty to hens if they feel desperate, i.e. when being regularly hounded by other males. No excuses in my mind, I cull them too.

Best wishes.