I would think molt or daylight issues would be the problem since it is the most typical reason this time of year.Decreasing day length and/or molt. Also check thoroughly for mites/lice. I have a headlamp and I lift their tails and check the vent area after dark.
Zanna~ My chickens would flip out if I tried to look under their tails LOL I wouldn't know whether to duck or run.....the chaos of flying chickens
