Color Variation and Hybridization among Nesospiza Buntings on Inaccessible Island, Tristan da Cunha

Abstract

Nesospiza buntings have speciated at the Tristan da Cunha archipelago in the central South Atlantic Ocean. Two species, the Tristan Bunting (N. acunhae) and Wilkins' Bunting (N. wilkinsi), differ markedly in size and co-occur without interbreeding on Night- ingale Island. Nearby Inaccessible Island supports two altitudinally segregated color morphs of N. acunhae, as well as a hybrid complex involving acunhae and wilkinsi. Plumage variation was evaluated for 581 buntings, most of which were caught on Inaccessible Island. Nesospiza plumages vary with age and sex; immature plumage is retained for at least two years. Despite age and sex differences, there is no overlap in coloration between the upland and lowland morphs of N. acunhae on Inaccessible Island. The two morphs are distinct from the time of hatching; chicks of lowland acunhae are pinkish, whereas upland acunhae are yellow. Color differences between acunhae morphs were quantitative rather than qualitative; their feathers contain the same suite of carotenoid pigments, but at three times greater concentration in upland birds. Dispersion of different bunting populations on Inaccessible Island is closely related to vegetation types. Individually marked individuals were largely sedentary; only immature birds moved more than 400 m. The parapatric dispersion of acunhae morphs prob- ably results from habitat-specific dietary differences. Nertera fruits are likely sources of ca- rotenoid pigments in the diets of buntings, but feeding experiments on captive birds are needed to elucidate fully the basis of color variation between morphs. Whatever the proximate cause, plumage-color variation may allow population differentiation if birds mate assortatively on the basis of color. A review of previous visits to the islands suggests that the diversity of