Why do chickens preen?

Andy Cawthray say:

Preening is an essential part of hygiene, and is often performed socially and invariably following a dust bath or when they have been caught out in the rain. The purpose of preening is to ensure proper feather structure is maintained by straightening the feathers (using the beak) and then oiling the feathers (using the head, first rubbed against the preen gland at the base of the tail and then rubbed over the feathers being preened). It is also a way of removing parasites, dead skin and other feather dander.

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