BirdsNRabbits
Chirping
Mine have black/brown dots/spots in their tails and/or wings as well.
Found a couple of links. Looks like it is called "peppering".
https://www.backyardchickens.com/t/826775/buff-orpington-with-black-speckled-tail-feathers
https://www.backyardchickens.com/t/379841/help-please-again-bo-w-dark-brown-black-tail-feathers
Some of mine would have some black speckles in their tails. During the fall moult they would usually lose them and grow back normal buff feathers. If you have hatchery stock (as I do) I think that's where that comes from.