Mites?

It could be mites and lice. Check them around their vents, too, and under their wings. Do you have any roosters? Roosters will pull out some head/neck feathers when breeding the hens. How old are they? When a chicken begins to molt, the head feathers are one of the first sets of feathers that a chicken drops. A chicken molts for the first time when it is about a year old, and molts every year thereafter.

 

How old are they? If older than a year (or about a year of age), then they could be molting. During molting, chickens lose their feathers, stop laying, develop paler combs/wattles, and during a period of several months will grow in new feathers. If not molting, then it could certainly be mites or lice. Check on the skin and feathers near the vent, under the wings, beneath the neck feathers, and near the base of the tail for moving black or red specks (mites), yellowish blobs (lice), or whitish things stuck to the feather shaft (lice eggs).

The only other thing I can think of is that your hens are picking on eachother. I would observe them for an hour or two, and watch for signs of feather pulling. If you have a rooster, the feather loss could also be due to overmating of the hens.

 

The red mite only comes out at night, so you would need to check after dark with a flashlight. Some chickens just like to pick each other's feathers, especially while on the roost--it's kind of like grooming each other. I have watched some of mine. To prevent feather-picking you can increase the protein from 16 to 20% or use some gamebird food for treats. Make sure they have plenty of room and some things to climb on and explore to reduce boredom. Things like a hanging cabbage or right now sitting a large pumpkin in the run for them to decorate would be good.