Grant most British butterfly watchers three wishes, and they would probably start the summer by visiting the Norfolk Broads to see the Swallowtail, Papilio machaon, glide majestically over the reed beds. Many would also choose a bird's-eye view of a southern oak wood in July, to marvel at rival males of the Purple Emperor, Apatura iris, engaging in furious dog-fights, flashing blue, black and purple high above their 'master tree'.