‘Gygis alba’ to be precise
Locally known as a "Seabird", St. Helena is a nesting site for the White Tern (Gygis alba), also confusingly referred to as the ‘Fairy Tern’ (by which name they are also known here).
Whatever you call them, they are a beautiful sight. If you are walking away from the towns and come near a nest they will hover around you, not apparently in a threatening way but more as if they are curious about this strange creature. In a shaded wood their (mis)name as Fairy Terns becomes apparent. Out in the open they perform spectacular aerobatic displays in pairs or threes (adults and juvenile), making seemingly impossibly sharp turns in perfect synchronisation.
They nest both inland, in the quieter wooded valleys (for example, near our home in Blue Hill) and also on the cliffs, even above the busy harbour in Jamestown. Balanced precariously on a narrow rock ledge or the branch of a tree it can be a wonder that the egg survives to become a chick and that the chick itself manages to constrain its wanderings to its tiny platform.