The gadfly petrels (genera Pterodroma and Pseudobulweria) are a cryptic, taxonomically complex group of 38 species predominantly found in tropical and temperate regions. Many are single island endemic breeders, often in remote areas, but all are wide-ranging at sea, with many being highly migratory and often occurring at low density over the pelagic abyssal plain. They range in size from 26 cm to 43 cm, with much interspecies plumage variation. Many species are polymorphic, and as a group they are notoriously difficult to distinguish, especially at sea.