COLOR VARIATION AND HYBRIDIZATION AMONG NESOSPIZA BUNTINGS ON INACCESSIBLE ISLAND, TRISTAN-DA-CUNHA

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 si ze and co-occur without interbreeding on Nightingale Island. Nearby In accessible Island supports two altitudinally segregated color morphs o f N. acunhae, as well as a hybrid complex involving acunhae and wilkin si. Plumage variation was evaluated for 581 buntings, most of which we re 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 up land and lowland morphs of N. acunhae on Inaccessible Island. The two morphs are distinct from the time of hatching; chicks of lowland acunh ae are pinkish, whereas upland acunhae are yellow. Color differences b etween 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 b unting populations on Inaccessible Island is closely related to vegeta tion types. Individually marked individuals were largely sedentary; on ly immature birds moved more than 400 m. The parapatric dispersion of acunhae morphs probably results from habitat-specific dietary differen ces. Nertera fruits are likely sources of carotenoid pigments in the d iets of buntings, but feeding experiments on captive birds are needed to elucidate fully the basis of color variation between morphs. Whatev er the proximate cause, plumage-color variation may allow population d ifferentiation if birds mate assortatively on the basis of color. A re view of previous visits to the islands suggests that the diversity of forms on Inaccessible Island could have been overlooked.