Geographical variation in the White‐tailed Tropicbird Phaethon lepturus, with the description of a new subspecies endemic to Europa Island, southern Mozambique Channel

Abstract

Geographical variation in measurements and colouration among populations and subspecies of White‐tailed Tropicbirds Phaethon lepturus, including birds from Europa Island (southern Mozambique Channel] was examined worldwide. Two groups were distinguished: the ‘large subspecies’(lepturus and fulvus from the Indian Ocean, catesbyi from the western Atlantic Ocean) and the ‘small subspecies’(ascensionis from the central and eastern Atlantic Ocean, dorotheae from the Pacific Ocean, and the birds from Europa Island). No clinal variation was found in the Indian Ocean, the birds from Europa Island being the only ‘small’ ones. This population also had a high frequency of golden morphs, a feature that does not exist elsewhere in the western Indian Ocean. These results indicate that Europa's population is isolated from all nearby colonies in the Indian Ocean, and does not belong to any of the two previously known subspecies of the area. It also differs from the birds of the two small subspecies by the frequency of the colour morphs and the distribution. Consequently, we propose to treat this population as a previously undescribed subspecies, endemic to Europa Island, for which we propose the name Phaethon lepturus europae. Geographical isolation of Europa Island and oceanic conditions in the Mozambique Channel are discussed to explain the isolation of this population.