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

Flight performance trade-offs and functional capacities of ruby-throated hu mmingbirds (Archilochus colubris L.) were studied using an integrative appr oach. Performance limits were measured by noninvasively challenging birds w ith two strenuous forms of flight: hovering in low-density gas mixtures (a lift assay for the capacity to generate vertical force) and fast forward ni ght in a wind tunnel(a thrust assay for the capacity to generate horizontal force). Functional capacities during hovering were measured by simultaneou sly collecting metabolic data using respirometry and information on wingbea t 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 reduc ed 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 capac ities in fast forward flight differed from theoretical models. Kinematicall y, hovering hummingbirds operate within a narrow range of wing-beat frequen cies, and modulation of aerodynamic forces and mechanical power is achieved primarily through variation in wing-stroke amplitude. Although differing i n hovering performance, both sexes. of nonmolt birds demonstrate similar me chanical and metabolic capacities, whereas molting inflicts high energetic costs. Relatively invariant physiological capacities may thus ultimately co nstrain the extent of intraspecific trade-offs between morphology and perfo rmance, providing mechanistic insights into the multilevel functional desig n of the hummingbird flight system.