Flightless rails endemic to islands have lower energy expenditures and clutch sizes than flighted rails on islands and continents

Data are presented on the standard energetics of six flighted and five flightless species of rails (Aves: Rallidae). The factors influencing these data and those from three additional species available from the literature, one of which was flightless, are examined. Basal rate of metabolism correlates with body mass, residency on islands or continents, volant condition, pectoral muscle mass, and food habits, but not with climate. The greatest capacity (96.2%) to account for the variation in basal rate of metabolism in 15 populations that belong to the 14 species occurs when body mass, volant condition, and food habits are combined. Then flighted species have basal rates that average 1.38 times those of flightless species and herbivorous rails have basal rates that are 1.37 times those of omnivorous species, which means that, independent of body mass, flighted gallinules have basal rates that are 1.9 times those of flightless, omnivorous rails. Distribution, pectoral muscle mass, and flight ability cannot be combined in the same analysis because they code for similar information. The evolution of a flightless condition in rails requires the absence of eutherian predators, but has occurred in the presence of marsupial predators. Each of the six studied flightless rails independently evolved a flightless condition and a low basal rate, whereas the evolution of herbivory and an associated high basal rate evolved at least twice in these species. Flightless rails on islands have clutch sizes that are only about one-half those of flighted rails living on continents, the reduction in clutch size correlating with a reduction in basal rate of metabolism. Thermal conductance in rails is correlated with body mass and food habits: herbivorous rails had conductances that were 1.43 times those of omnivores, i.e., conductances are highest in species with the highest basal rates.