Hummingbirds of the Juan Fernández Islands: natural history, evolution and population status

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Abstract
Two hummingbird species inhabit the Juan Fernández Islands, nearly 700 km off the Chilean coast in the Pacific Ocean—the endangered endemic Juan Fernández Firecrown Sephanoides fernándensis and the continental Green‐backed Firecrown S. sephaniodes. In terms of body size, the endemic species is the most sexually dimorphic species of hummingbird known; it also displays an extraordinary degree of sexual dichromatism. Both sexes hold feeding territories, within which courtship probably occurs. It is suggested that sexual selection, and selection for gender recognition and the absence of indigenous predators may explain the evolution of sexual dimorphism and dichromatism in the Juan Fernández Firecrown. In spite of a more than twofold difference in body size, the bill lengths of both sexes in both species are nearly identical and closely match the flower tube length of the several species of endemic plants they pollinate.The endemic Juan Fernández Firecrown has become extinct on one of the two main islands (Isla Alejandro Selkirk) and its population on the other main island (Isla Róbinson Crusoe) has greatly declined in recent decades. In contrast, the population of the Green‐backed Firecrown has probably increased on Róbinson Crusoe and the species has recently become established on Alejandro Selkirk. Because historical records show that Green‐backed Firecrown survived centuries of potential competition from Juan Fernández Firecrown, massive habitat destruction, plagues of rats, feral cats and dogs and the effects of feral livestock before beginning its recent decline, it is suggested that the introduction earlier in this century of the bramble Rubus ulmifalius, the coati Nasua nasua and possibly the rabbit Oryctolagus cuniculus may have differentially favoured the Green‐backed Firecrown at the expense of the Juan Fernández Firecrown.