You have no idea how much I wish someone knew what was wrong with your birds because mine look just the same. I've treated endlessly for mites and now and going to deworm them, although I'm convinced that is not the problem. I can guarantee it's not feather picking, not the way the whole body is affected. And I've never seen a mite on any of my hens. I've been in a quandry for months on end. My only hope now is that they will molt and whatever was the problem is gone and they'll grow nice new feathers. I hope! Good luck and please let me know if you ever come up with a solution. Thanks!
So long as you're certain its not lice, mites or an over zealous roo I did find the below information while reading up on the 5 types of feathers chickens have:
" Good feather growth and maintenance requires adequate amounts of proteins, amino acids, minerals and vitamins. No matter how much a hen may preen, if good management practices are not employed, the feather quality of your birds is likely to be poor.
There are, however, three exceptions to this.
Firstly, the very good layers in your flock are likely to have a tattered appearance, displaying broken and brittle feathers — the hen is converting the protein in her food to the production of eggs rather than pretty feathers.
Secondly, if you keep a rooster, you may find that he has one or two ‘favourite’ hens. These hens are likely to display brittle and broken feathers caused by the ‘extra’ attention they seem to attract. In such circumstances, it’s best to move the hens to a rooster-free zone.
Finally, just before and during moulting you will notice that the feather quality is poor.
Providing best management practices and ensuring your hens have an adequate and balanced diet, water and by minimising stressful conditions in your flock will help to ensure strong feathers which your hens will take great delight in keeping clean! "