Chances are you older birds have already ramping down production and are getting ready to molt due to the shortening days.
First find out if they have lice and/or mites, then figure the best way to treat for them:
Check them over real well for mites and/or lice.
Google images of lice/mites and their eggs before the inspection so you'll know what you're looking for.
Part the feathers right down to the skin around vent, head/neck and under wings.
Best done well after dark with a strong flashlight/headlight, easier to 'catch' bird and also to check for the mites that live in structure and only come out at night to feed off roosting birds.
Wipe a white paper towel along the underside of roost to look for red smears(smashed well fed mites).
Good post about mite ID by Lady McCamley:
https://www.backyardchickens.com/threads/my-chicken-has-mites-now-what.1273674/page-2#post-20483008
Here's how to add your general geographical location to your profile.
It's easy to do, and then it's always there!
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