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My girls' feathers all look like that around their preen glands - a bit dirty and oily.

Looks just like the preen gland to me! The 'biting' is actually them transferring the oil onto their beaks in order to preen it over the rest of their feathers. Let's just check - is it on the saddle, where the back meets the tail feathers? If so, it's totally normal, and indeed totally necessary!