Dust bathing is an important activity for chickens and for many other birds. They scrape up dust with their claws, then scrounge around it in, ruffling their feathers and contorting themselves into all kinds of curious poses in the process. The activity cleans feathers of dandruff and oils (basically everything shampoo claims to do for humans) and it is thought to aid in external parasite removal. But I’m pretty sure it also serves some sort of social function, since I never see chickens dust bathing on their own. What specific social roles dust bathing plays I don’t know, but if my chickens want a spa treatment, I’m not going to deny them.
On our farm we want to allow each animal species to express its instinctive behaviors. So pigs root instead of standing on concrete, cattle graze instead of eating from a trough, and chickens take dust baths. Obviously there are limits we place on some instinctive behaviors (like building fences so our bull can’t sally forth to wage war with other bulls in the neighborhood), but in our philosophy the less we constrain the animals, the better they are being treated.
The ability to dust bathe is something I see as a welfare concern. Cage-raised chickens can’t do it, even though they try. Even commercial “free range” chickens (i.e., chickens raised in buildings but not in wire cages) usually can’t bathe because the litter material on the ground is too chunky or fibrous for their needs. (To be fair, chickens who live indoors could be provided with dust baths, but if you look at commercial operations you’ll rarely find this provision.) In the sales pitch for buying eggs from pasture raised hens, I see this as a big deal. Among other things, hens on pasture are free to dust bathe.
Our hens scratch up dirt even when it is frozen hard. But when they are snowbound, we’ve found that they are quite satisfied using the pigs’ feed as a substitute. As long as there is a dusty medium, it doesn’t seem to matter. They are content. And we’re content if the they are.