Fig. 5. Discrimination between feeders with hyperoxia-filled (filled squares) or normoxia-filled (open squares) corollas during failure and the two feeding bouts just prior to failure of hovering. Values are the mean incidence ±1 S.E.M. of choice for hummingbirds at that density (N=7). Birds that sampled both feeders during one feeding bout received a score of 1 for both feeders. Data are from the hummingbirds used in the choice experiment.