ENERGY-CONSERVATION AND THE EVOLUTION OF FLIGHTLESSNESS IN BIRDS

I examine the hypothesis that energy conservation contributes to the e volution of a flightless condition in birds by comparing the factors t hat correlate with basal rate of metabolism in kiwis and flighted and flightless rails and ducks. Flightless rails have low basal rates, the level of which decreases with pectoral muscle mass. Kiwis also have l ow basal rates and small pectoral masses. The small pectoral masses fo und in flightless grebes, the flightless cormorant, and the flightless parrot suggest that these species have low basal rates. Penguins and flightless ducks, in contrast, have neither low basal rates nor small pectoral masses because these birds use their wings for locomotion. Th ese data are compatible with the hypothesis that energy conservation c ontributes to the evolution of flightlessness in species in which pect oral muscle mass is reduced. On oceanic islands, rails have evolved a flightless condition repeatedly, usually in association with a small b ody size. Both adjustments reduce energy expenditure, which thereby fa cilitates the persistence of rails in environments with limited resour ces. The evolution of flightlessness in insects may also be a response to a restricted resource availability, especially in persistent habit ats characterized by low rates of production.