It's possible they are going into a mini molt, maybe even a full molt. My EE loses part of her muff about this time of the year and it drives me nuts thinking she has lice or mites...and she doesnt. Last year my Red Star lost feathers all the way around her upper neck and head...she almost looked like a red vulture lol.
It wasnt until this past February that my EE finally grew out a nice, full beautiful muff. There's no telling when their feathers will regrow, but you can check and see if any pin feathers are starting to grow in and around the balding areas. For these 2 particular hens, you can increase their protein for awhile to help them with feather regrowth...but i wouldnt do it on a long term basis. BTW, nice looking Black Star. They and Barred Rocks are my favorites.