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This study shows that the Red-tailed Tropicbird (Phaethon rubricauda) has a gradual clinal increase in the intensity of the rose-pink suffusion, egg size, culmen length and wing length in breeding populations between Kure Atoll in the northern Pacific and the Kermandec Islands in the southern Pacific. The illusion that birds from this cline comprise three subspecies has long been accepted because the large range of latitude that each subspecies had been arbitrarily given resulted in significant differences in mean measurements. However, as neither the northern rothschildi nor the southern roseotincta are clearly separable from melanorhynchos in the centre of the cline, they must all be one subspecies. Because the mean measurements of the nominate “subspecies” are nor significantly different from those of birds from similar latitudes in the Pacific cline, or from westralis in the eastern Indian Ocean, there is no valid reason for distinguishing any subspecies in the Red-tailed Tropicbird.