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.