I agree, depluming mites would cause that sort of damage. They are microscopic, so you won't see them. I think perhaps the Ivermectin treatment needed to be repeated, but also the feathers will not look any better until they are moulted and replaced which may now not be until next year.... have you seen them moult in the past few months? I agree, pour on Ivermectin would be the best option. If you know any farmers, they may let you have the tiny amount you would need as it is sold for cattle and sheep so in pretty large quantities. It's effectiveness will best be assessed by the birds' behaviour rather than their feather quality because as we have mentioned, the feathers will not improve until they moult.
