Are your ducks white or light colored @CherpNPeep my white Muscovys I could always see their yellow oil from their preen gland on some of their feathers.
So that's why my Indian Runners white sweat-shirts (chest) become yellowish-orange when the weather is cold! They preen like crazy and i was asking myself if there's something wrong with my eyes…I've seen the same on Pekins.
I have learned something today! Thank you very much!