Young Brunette Girl With A Blue Feather

Feather holes are chewing traces of lice (most probably Brueelia spp. lice) on the wing and tail feathers. They were described on barn swallows, and because of easy countability, many evolutionary, ecological, and behavioral publications use them to quantify the intensity of infestation.
Interestingly, parasitic cuckoos which grow up in the nests of other species also have host specific feather lice and these seem to be transmitted only after they leave the host nest.
Birds maintain their feather condition by bathing in water, dust bathing and preening. A peculiar behavior of birds, anting, where ants are introduced into the plumage was suggested to help in reducing parasites but no supporting evidence has been found.