Maybe treat it like wet feather it's rec to use Dawn soap. you know babies can get something called cradle cap wonder if this is similar.
http://www.majesticwaterfowl.org/artwetfeather2.htm
http://www.majesticwaterfowl.org/artwetfeather2.htm
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Cradle cap is exactly what I was thinking of.Maybe treat it like wet feather it's rec to use Dawn soap. you know babies can get something called cradle cap wonder if this is similar.
http://www.majesticwaterfowl.org/artwetfeather2.htm
I am wondering if it's the preening gland being too active? The patterning of the grease you described would be consistent with this. I would be shocked if an avian vet missed that though! Hopefully the U will have some answers.