Hovering performance of hummingbirds in hyperoxic gas mixtures

Hummingbirds evolved during a period of decline in atmospheric oxygen conce ntration and currently encounter varying levels of oxygen availability alon g their elevational distribution. We tested the hypothesis that inspiration of hyperoxic gas increases hummingbird hovering performance when birds are simultaneously challenged aerodynamically. We measured the maximum duratio n of hovering flight while simultaneously monitoring the rate of oxygen con sumption of ruby-throated hummingbirds (Archilochus colubris) in low-densit y heliox that was either normoxic (21% O-2) or hyperoxic (35% O-2) AS air d ensity decreased below 0.85 kg m(-3), hummingbirds hovered significantly lo nger in hyperoxia than in normoxia, but the air density at which the birds could no longer sustain hovering flight was independent of oxygen concentra tion. At low air densities in hyperoxia flight trials, hummingbirds appeare d to increase their rate of oxygen consumption relative to flight sequences at equivalent densities in normoxia trials, but these differences were not significant. We tested the hypothesis that hummingbirds can discriminate b etween environments that differ in oxygen concentration. In another density -reduction experiment, hummingbirds were allowed to choose between artifici al feeders infused with either normoxic or hyperoxic gases. The hypothesis was not supported because birds failed to associate oxygen concentration wi th a particular feeder independently of air density. Supplemental oxygen th us yields increased hovering duration at intermediate air densities, but th e minimum density at which birds can fly is limited exclusively by aerodyna mic considerations.