Thanks for the replys and welcomes.
I have had a closer look at the flock and have found it in at least 4 other birds, though not as bad as this one.
It makes me think that it is something systemic.
But the perch is not too high, only about 8 inches, and the house is cleaned once a week.
I have sterilised the house with a flame weeder, including the perch.
I have also isolated the worst affected girl, to try and reduce the risk of further spread.
Any other ideas as to why there are so many with this affliction?
A point to note, I originally had 22 girls, who are now 1.5 years old, and added another 20 pullets about a month ago. Pecking order chaos ensued, but did not seem to be too brutal. It was the original older girls that seemed to be most put out by the experience, even though it was them harrassing the new girls.
And all of the afflicted girls are the old girls!
Could it be that the stress of the new additions has weakened their immune system?
Any comment welcome.