LIMITS TO FLIGHT ENERGETICS OF HUMMINGBIRDS HOVERING IN HYPODENSE ANDHYPOXIC GAS-MIXTURES

Hovering hummingbirds offer a model locomotor system for which analyse s of both metabolism and flight mechanics are experimentally tractable , Because hummingbirds exhibit the highest mass-specific metabolic rat es among vertebrates, maximum performance of hovering flight represent s the upper limit of aerobic locomotion in vertebrates, This study eva luates the potential constraints of flight mechanics and oxygen availa bility on maximum flight performance, Hummingbird flight performance w as manipulated non-invasively using air and gas mixtures which influen ced metabolism via variable oxygen partial pressure and/or altered fli ght mechanics via variable air densities, Limits to the locomotor capa city of hovering ruby-throated hummingbirds (Archilochus colubris) wer e unequivocally indicated by aerodynamic failure in either air/helium or air/heliox mixtures, Air/helium mixtures are hypodense and hypoxic; failure to sustain hovering flight occurred at 63% of the density of sea-level air and at an oxygen concentration of 12%, Air/heliox mixtur es are hypodense but normoxic; failure in hovering occurred at 47% of sea-level air density, Thus, hummingbirds demonstrated considerable po wer reserves in hovering flight as well as hypoxic tolerance, In air/h elium mixtures, hovering was limited by oxygen supply and not by fligh t mechanics, Birds hovering in air/helium mixtures increased their mec hanical power output but not their rate of oxygen consumption, By cont rast, birds hovering in air/heliox mixtures increased both mechanical performance and metabolic expenditure, Under hypoxia, hovering humming birds demonstrated non-negligible, but still limited, capacities for a naerobic metabolism and/or oxygen storage, Depending on the physical c ontext, hummingbird flight performance can therefore be limited by oxy gen availability or by flight aerodynamics.