Does the metabolic rate-flight speed relationship vary among geometrically similar birds of different mass?

Based on aerodynamic considerations, the energy use-flight speed relationship of all airborne animals and aircraft should be U-shaped. However, measures of the metabolic rate-flight speed relationship in birds have been available since Tucker's pioneering experiments with budgerigars nearly forty years ago, but this classic work remains the only study to have found a clearly U-shaped metabolic power curve. The available data suggests that the energetic requirements for flight within this species are unique, yet the metabolic power curve of the budgerigar is widely considered representative of birds in general. Given these conflicting results and the observation that the budgerigar's mass is less than 50% of the next smallest species to have been studied, we asked whether large and small birds have metabolic power curves of different shapes. To address this question we measured the rates of oxygen uptake and wingbeat kinematics in budgerigars and cockatiels flying within a variable-speed wind tunnel. These species are close phylogenetic relatives, have similar flight styles, wingbeat kinematics, and are geometrically similar but have body masses that differ by a factor of two. In contrast to our expectations, we found the metabolic rate-flight speed relationship of both species to be acutely U-shaped. We also found that neither budgerigars nor cockatiels used their normal intermittent flight style while wearing a respirometric mask. We conclude that species size differences alone do not explain the previously unique metabolic power curve of the budgerigar; however, due to the absence of comparable data we cannot evaluate whether the mask-related kinematic response we document influences the metabolic rate-flight speed relationship of these parrots, or whether the energetics of flight differ between this and other avian clades.