The first time I caught my chickens at their bathing routine, I thought they were dying. They looked like they were in the grip of a seizure of some kind.
In a panic, I ran to get my Dad, the expert on chicken behavior. He laughed himself sick.
Once he caught his breath, he told me that this is how chickens like to bathe, in dirt. I couldn't believe that they would choose to bathe in soft dry dirt, such as that under a big cedar tree.
The thought was almost too much for me.
I took to spying on my bantam chickens, to see if I could catch them at it, and it seems that they have a routine too. Generally, they would scratch around for some bugs, lay an egg, then head for the dry dust under the tree, their favorite spot. Sifting the dirt through their feathers, they would scratch with their feet, and then roll in the soil.
I was eleven years old.
The way chickens clean themselves has never lost its appeal for me, and setting things up in a chicken coop gave me the opportunity to figure out how to provide the type of bath they like.
An old tire, set in a corner, with the addition of some clay like dusty soil, or even wood ashes, was their go-to place to get rid of bugs, scratch their itches, and preen afterwards.
The preening is really important, both to re-set the feathers, and to oil them. There is a gland just above their tail, which provides the oil. If you watch them, they'll get a little bit of the oil on their beak and then put it where it needs to go among their feathers.
Bathing and preening is a very social event in a chickens life, and you'll often see them in groups engaging in these activities.
In a pen outdoors, make a roof of some kind - keep in mind that they will climb on it, so make it sturdy enough for that. The bathing material has to be dry - they will try to use it anyway, and get even dirtier if it's wet or damp.