I find it interesting when people attribute abstract intentions to this kind of behaviour. The chick's probably not thinking "man I gotta dust myself for mites", it probably just feels good to get dusty. Now, the reason it feels good is probably that dust baths are correlated with fewer parasites and better overall outcomes, but I doubt the chick knows that.

Same thing with cats liking scritches under their chin. They're not thinking "ah yes I am scent marking this human", scent glands are probably just itchy.