HUMMINGBIRD HOVERING PERFORMANCE IN HYPEROXIC HELIOX - EFFECTS OF BODY-MASS AND SEX

Owing to their small size and hovering locomotion, hummingbirds are th e most aerobically active vertebrate Can hyperoxia enhance the flight of this highly oxygen-dependent group? Hovering performance of ruby-th roated hummingbirds (Archilochus colubris) was manipulated non-invasiv ely using hyperoxic but hypodense gas mixtures of sea-level air combin ed with heliox containing 35 % O-2. This manipulation sheds light on t he interplay among metabolic power input, mechanical power output and aerodynamic force production in limiting flight performance. No signif icant differences in flight mechanics and oxygen consumption were iden tified between hyperoxic and normoxic conditions. Thus, at least in th e present experimental context, hyperoxia did not change the major met abolic and mechanical parameters; O-2 diffusive capacities of the resp iratory system were probably not limiting to a significant extent. Com pared with hummingbirds in our previous studies, the present experimen tal birds were heavier, had resultant shorter hover-feeding durations and experienced aerodynamic failure at higher air densities. Because h ummingbirds have relatively stable wingbeat frequencies, modulation of power output was attained primarily through variation in stroke ampli tude up to near 180 degrees. This result indicates that maximum hoveri ng performance was constrained geometrically and that heavier birds wi th greater fat loads had less margin for enhancement of power producti on. Sexual dimorphism in flight adaptation also played a role, with ma les showing more limited hovering capacities, presumably as a trade-of f for increased maneuverability.