When chooks start to feel a little bit dirty or gritty, they will dig a shallow ditch (in soil, mulch, sand, or other dry, loose materials), in which they will burrow and throw the dirt over themselves. They will do this until the dirt has coated their feathers and is settled down to their skin- where it is believed to absorb excess moisture and oil, and clog the breathing pores of parasites that may have found their way onto your chicken. They will then stand up and shake out all the dirt- so be sure to be standing away from the action, they then proceed to preen themselves before getting on with their usual chicken business.
"Sham dustbathing occurs when the motivation for certain behaviour builds to a sufficiently high level that the behaviour is performed in the complete absence of relevant stimuli." (1)
Unfortunately caged hens are denied the ability to express this innate behaviour. Being housed in four sided wire cages void of any substrate. With an allocated space per bird equivalent to that of an A4 piece of paper, they are barely able to turn. Yet many of these condemned hens still will "Sham dust bath".
Could you imagine being denied the ability to bathe for the entirety of your life? Even if you stayed indoors just as these hens do your skin would soon start to itch and become unbearable. All because human beings feel that an egg should be cheaper than water! It is NOT OK to deprive any living species of it's natural behaviours in the name of human greed! Eggs are not hard to produce outdoors, where chickens get to be chickens. Australia has the land mass to produce eggs the way nature intended.
(1) Lindberg, A.C. and Nicol, C.J. 1997. Dustbathing in modified battery cages: is sham dustbathing an adequate substitute? Applied Animal Behaviour Science, 55: 113–128