Limits to exercise performance are little understood in animals. Hummingbirds exemplify metabolic specialization for enhanced energetic capacity during flight, and thus represent a prime example of extreme physiological design in respiratory, cardiovascular, and muscular systems. The analysis of hummingbird flight may thus reveal fundamental limits to the biomechanics and physiology of vertebrate locomotor performance, including that of humans. This study will examine limits to flight performance in diverse hummingbird species, and will determine potentially variable contributions of body size, wing morphology, flight kinematics, aerodynamics, and respiratory energetics to flight capacity. Interspecific comparisons of hummingbirds will include studies of hovering flight performance across altitudinal gradients which, because of physical variation in air density and oxygen availability, present particular aerodynamic and physiological challenges to flying animals. Analysis of hovering flight will be augmented by study of limits to forward flight performance carried out on hummingbirds flying at different airspeeds in a wind tunnel. The combined results for hovering and forward flight will yield basic conclusions as to the nature of aerodynamic and physiological constraints on hummingbird flight capacity, and more generally will delineate broad performance limits to vertebrate exercise physiology.
