They look a very happy flock in their lovely new coop, Ed. Plenty of space, so less chance of redmite from overcrowding, especially as its new wood, and the weather has been so cold so I wouldn’t think a major outbreak of redmite is very likely yet.
I think your idea about stress may be possible, though. It sounds as if you were down to two hens, Tikka and another, then introduced 4 more at about the time she stopped laying and the feather loss started. How did she react when the newbies arrived, having to re-establish her position as top hen? Introductions are always a cause of stress, both to new young pullets and older established birds, and after only three weeks together the flock will still be in flux and will take a while to come to understand their new dynamics. It’s good that they have plenty of space to work it out for themselves.
It’s still possible, of course, that she has some sort of infestation, and a dose of lice powder on all of them would do no harm. Also, when was the last time you wormed the older ones with Flubenvet, and have you done the new ones yet? It’s good to do pullets arriving from a different biosystem once they’ve settled in well enough to be eating properly, to deal with any worms they may be importing from a breeder’s premises where hens have been kept on the same ground for a long time. There’s advice on here about how to do this by using Flubenvet powder and mixing it into a treats mix, which is better for small numbers than buying a premix bag of treated pellets, as you’ll probably either not have enough for them all for the week, or will have some over that will be out of date when next needed. A pot of powder will have enough to treat 20 hens, so will do your 6 three times over.
