Maximum flight performance of hummingbirds: Capacities, constraints, and trade-offs

Flight performance trade-offs and functional capacities of ruby-throated hummingbirds (Archilochus colubris L.) were studied using an integrative approach. Performance limits were measured by noninvasively challenging birds with two strenuous forms of flight: hovering in low-density gas mixtures (a lift assay for the capacity to generate vertical force) and fast forward night in a wind tunnel(a thrust assay for the capacity to generate horizontal force). Functional capacities during hovering were measured by simultaneously collecting metabolic data using respirometry and information on wingbeat kinematics for aerodynamic analysis. Intraspecific differences in flight capacity, presumably reflecting diverse selective forces because of sexual dimorphism, migration, and plumage renewal, were then compared. Birds with Shorter wings (adult males) or with increased body weight displayed a reduced hovering capacity, although their maximum flight speed was unaffected by such morphological changes. Birds undergoing molt of their flight feathers exhibited a diminished performance during both hovering and forward flight. Hovering capacities in relation to variation in wing morphology and body mass were congruent with aerodynamic predictions, whereas performance capacities in fast forward flight differed from theoretical models. Kinematically, hovering hummingbirds operate within a narrow range of wing-beat frequencies, and modulation of aerodynamic forces and mechanical power is achieved primarily through variation in wing-stroke amplitude. Although differing in hovering performance, both sexes. of nonmolt birds demonstrate similar mechanical and metabolic capacities, whereas molting inflicts high energetic costs. Relatively invariant physiological capacities may thus ultimately constrain the extent of intraspecific trade-offs between morphology and performance, providing mechanistic insights into the multilevel functional design of the hummingbird flight system.