A Pigeon is left for Roger to release at Strickland Junction in Pigeon Post. Nancy says that Uncle Jim gave us one, and called him Homer because he was a homing pigeon. They got two more for company, which they called Sophocles and Sappho after looking up Greek poets (PP1). When they go camping, Dick has to make the returning pigeon ring a bell to alert Mrs Blackett (PP6).
At the end, though Titty says that you cant count on Sappho, Sappho carries the message which calls out Colonel Jolys and the firefighters (PP31).