Limits to vertebrate locomotor energetics suggested by hummingbirds hovering in heliox

Abstract

OXYGEN consumption¹ and muscle power output² of hovering hummingbirds are among the highest recorded for vertebrates. Maximum performance of hummingbirds thus approaches the upper limits of vertebrate aerobic locomotion³. Because air density is a major determinant of aerodynamic power requirements⁴, hovering flight performances⁵ can be manipulated non-invasively using nor-moxic gas mixtures of variable density⁶. Here we show that limits to the locomotor capacity of hovering ruby-throated hummingbirds are unequivocally indicated by aerodynamic failure at low densities less than half that of sea-level air. Hummingbirds demonstrate considerable power reserves, with muscle mass-specific power (assuming perfect elastic energy storage) averaging from 98 W kg^-1 in normal air to a maximum value of 133 W kg^-1 before aerodynamic failure. In contrast to such variable power expenditure, however, muscle efficiency remains approximately constant at 10%. Modulation of power output is attained primarily through variation in wing-stroke amplitude, with aerodynamic failure occurring near 180°.