Basal Metabolic Rate of the Black-Faced Sheathbill (Chionis minor): Intraspecific Variation in a Phylogenetically Distinct Island Endemic.

Metabolic rate is a fundamental characteristic of all organisms. It covaries most significantly with activity, body mass, seasonality, and temperature. Nonetheless, substantial additional variation in metabolic rate, especially either resting rate or basal rate, is associated with a range of factors including phylogenetic position, ecological distinctiveness, range position, and diet. Understanding this variation is a key goal of physiological ecology. The black-faced sheathbill is a phylogenetically distinct, high-latitude, island-endemic bird occurring exclusively on several archipelagos in the southern Indian Ocean. Here we examined the idea that the unique phylogenetic position and ecology of the black-faced sheathbill may lead to a basal metabolic rate (BMR) different from that predicted by its body mass. When compared with BMR data available for all birds and a subset of island species, it was clear that the BMR of the black-faced sheathbill on subantarctic Marion Island, estimated at 15°C using indirect calorimetry (2.370 ± 0.464 W, mean ± SD; n = 22), for a group of birds with a mean mass of 459 ± 64 g, is no different from that expected based on body mass. However, variation in BMR, associated with habitat use and diet, even when correcting for variation in mass, was found. Sheathbills foraging year-round in comparatively resource-rich king penguin colonies have a higher BMR (2.758 ± 0.291 W, n = 12) than sheathbills that split their foraging between rockhopper penguin colonies and the intertidal zone (2.047 ± 0.303 W, n = 10), which are poorer in resources. Because these populations coexist at relatively small spatial extents (the entire island is 290 km(2)), other factors seem unlikely as causes of this variation.