They may not have ever seen real dirt yet in their lives. Give them time. They may also have to learn it from the others. I am assuming these were not raised by a mum, otherwise mum would have told them.See, this upsets me. She's dust bathing in a millimetre (if that) of woodshavings when there's a nice deep pile of dirt right there.
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The Rascals never used the dirt bin I gave them in Hattie House preferring to dustbath in the straw. It was only once they moved to the Cluckle Hut and the big run that they started using dirt to dustbathe. I don't know if they saw the others doing it or if it was the fact that there were wonderful dust holes waiting for them to use or if it was because once they were down the ramp, they did not go back into the Hut so all there was to use was dirt. It could be any of those reasons.