Bird feathers are made same material as hair, Keratin. Under those feathers, a bird is hidden. That bird needs to stay clean. 

Their skin is constantly regrowing and replacing itself, and this helps repel microorganisms from living on the skin. New skin cells grow up from the lower skin layers, and old skin cells flake off at the surface and end up mixed into the feathers, next some types of birds have to powder down a specialized type of feather,

Now, powder down feathers breaks into little fragments to make the other feathers waterproof. Those fragments build up over time and are mixed into the feathers. Most birds without powder down have preen oil,

they keep their feathers waterproof by spreading oil all over their feathers. They later eat some of that oil to get vitamin d, but the rest builds up in the feathers and whatever dirt or dust sticks to it. 

There are also mites and other very small animals completely adapted to live in bird feathers. Some are harmless and eat the feathered dust and oil, and some are parasitic and feed on blood.

How to solve this?

Either way, the bird needs to keep these under control. Birds have found a way to solve some or all of these problems and at the same time,

The bath.

Birds can bathe by standing in water and dunking their feathers, and they ruffle open all their feathers to let water get in towards their skin, they flick water up on their backs, or like this goose, they get their backs wet more directly,

Why do birds take dirt baths?

If a bird can’t get to a pool of water, it will also take dust baths, making very similar motions to get dust between the feathers, then shake it out. There are several theories as to what bathing does. Water is certainly rinsing away feather particles and loose skin. Dust probably does the same thing by binding with oil and making clumps that the bird can shake away.

Water or dust probably makes life harder for mites. Hopefully, it washes them away, but at the very least, it will make them wet or dusty and slow them down. But how common bird baths are, there

has been no scientific explanation that has locked down exactly why birds take baths. We can speculate for now. If you’re looking for a research question about birds, this one is just waiting for you to find the answer.

To know more about various bathing and feather cleaning technique birds follow click here.

If you are making bird baths at home,

How to make a good dust bath? 

They love peat moss. 

the peat moss is light and fluffy, so it easily glides between their feathers and skin. And also, add compost to coat them in beneficial microbes and perlite to have a coarser material that takes off those big chunks.

So this winds up creating a super-powered mixture that the birds love.