Q: What is a dust bath, and do my chickens need one?
A: A dust bath is essentially a "bath" your birds will take in dry dirt. They'll flop around looking as if they're dying. But don't be alarmed! They're okay. They might even be blissfully happy.
They're tossing dust into their feathers, which helps to repel external parasites like mites or lice. If you're wondering how they get a dust bath, well... they'll create their own in most cases! If they have access to an outdoor space, they'll find a corner or go under a bush and wallow out their own dust bath.
You can also provide your chicken with a dust bath, if you prefer. You might want to do this if your chickens don't have access to a yard--for instance if you live in town and have a small covered run filled with bedding rather than on the ground with dirt or dust. You may also want to provide your chickens with a dedicated dust bath if you want to spare your landscaping or lawn! Your flock will not consult you before wallowing, so they may choose inconvenient spots.
A child's sandbox makes a nice dust bath; it's low and not tippable. You can also use a low-sided plastic storage tub, but they do tend to tip over as your flock perches on the sides, awaiting their turn. You can also build one like you might build a raised garden bed.
Whatever you choose to make your dust bath with, you'll probably want to have it in a sheltered area so the dirt/dust stays dry. (There is a difference between dust bathing and mud bathing! Chickens prefer the former.
So... do chickens absolutely need a place to dust bathe? Yes and no. No, in that they'll survive without a spot. Having no dust bath is not an existential threat the same way having no food or water would be. They WILL bathe in their bedding if need be, but that is not ideal and just won't be as effective as dry, dusty dirt. But that being said, without a dust bath, one of their natural, instinctual behaviors will be frustrated. They'll be less happy. They'll also be more vulnerable to mites and lice.
You can make your own dust bath mix by combining 10 parts playsand with 1 part each wood ash and diatomaceous earth. You can throw in some dried lavender---said to help repel insects. We're not sure how much function the lavender adds, but it won't hurt anything (and it makes us feel fancy!). Keep in mind that this isn't baking... the exact proportions of the dust bath recipe needn't be followed precisely. You simply want your mix to be dry and dusty without creating a choke-inducing cloud. If you live in a low-humidity environment, for instance, you may find you need to reduce your ash and DE so the mix is not quite as dusty. In very high humidity, you may want to increase the sand.