A) Almost all swifts carry bloodsucking louse flies called Crataerina pallida, a species of flat fly that is found exclusively on swifts. These tough flies are almost as big as a penny coin, and their only food is the blood that they suck from their host. They remain on the swifts throughout their lives, so have an extraordinary life cycle, only coming into contact with earth when the swifts come back here to breed.
Though many people think that a high infestation of these flies must be detrimental to the swift’s health, there is no evidence that this is the case. Indeed, the fly depends on the swift for its survival, so it would not want to kill it. Whether the swift gains any advantage from the fly is unproven, though it seems doubtful.