FLIGHT THERMOGENESIS AND ENERGY-CONSERVATION IN HOVERING HUMMINGBIRDS

As the smallest homeotherms, hummingbirds suffer from low thermal iner tia and high heat loss. Flapping flight is energetically expensive, an d convective cooling due to wing and air movements could further exace rbate energy drain. Energy conservation during flight is thus profound ly important for hummingbirds. The present study demonstrates that hea t produced by flight activity can contribute to thermoregulatory requi rements in hovering hummingbirds. The rate of oxygen consumption, as a n indicator of metabolic cost, was measured during hover-feeding and c ompared with that during perch-feeding. In hover-feeding, oxygen consu mption increased only moderately between 35 and 5 degrees C in contras t to the sharp increase during perch-feeding over the same temperature range. This result suggests that heat produced by contraction of the flight muscles substituted for regulatory thermogenesis to accommodate for heat loss during temperature. With declining air temperatures, th e mechanical power requirements of hovering decreased slightly, but me tabolic costs increased moderately. As a result, the mechanical effici ency of the muscle in converting metabolic power to mechanical power w as I educed. Changes in wingbeat kinematics also accompanied the reduc tion in muscle efficiency. Wingbeat frequency increased but stroke amp litude decreased when hovering in the cold, suggesting thermoregulator y roles for the flight muscles. Hovering hummingbirds modulated their wingbeat frequency within a narrow range, reflecting the physical cons traints of tuning to a natural resonant frequency with an elastic rest oring force. We hypothesize that, by forcing the resonant system of th e wings and thorax to oscillate at different frequencies, muscle contr action in the cold generates more heat at the expense of mechanical ef ficiency. This mechanism of modulating the efficiency of muscle contra ction and heat production allows flying hummingbirds to achieve energy conservation at low air temperatures.