Serious consequences
Some of the parasites damage feathers by feeding on them, others suck the birds' blood. This reduces the ability of the birds to fly, retain body heat, survive and reproduce.
Bird warfare
Birds preen their feathers constantly in order to get rid of parasites. This is an important measure, as the parasites are very common and far from harmless. They cause almost as much death among young chicks as do predators. House sparrows exhibit similar remarkable behaviour when they "bathe" in sand.
