This is because there are feather lice on the pigeon's feathers.
It is also a reminder that the dovecote needs to be cleaned properly. It's too dirty and the pigeons need to be bathed and tanned regularly, so they can better avoid parasites.
You can buy cockroach or ant poison from a place that sells daily chemicals, then apply it evenly to the pigeons' feathers. Then find a small cage, the smallest, or a box, just large enough to put the pigeons in it. Then lock the pigeons inside for about two hours. After doing this for two or three consecutive days, all the feather lice on the feathers will basically be cleaned off.