Meeting Abstract
Flying hummingbirds generate large amounts of endogenous heat due to the mechanical inefficiency of their flight muscles. Feathers must lie flat during flight to minimize drag, restricting heat dissipation to specific heat dissipation areas (HDAs) around the eyes, shoulders, and feet. When temperature is moderate (23°C), excess heat can be dissipated passively through HDAs to maintain heat balance. However, when temperatures become high, passive heat dissipation is likely reduced due to the loss of a thermal gradient, requiring increased dependence on evaporative heat dissipation. To determine if calliope hummingbirds (Selasphorus calliope, 2.5g) can maintain heat balance during hovering at higher temperature, we used infrared thermography, open-flow respirometry, and particle image velocimetry (PIV) to model their heat budgets at low Ta (~23 °C) and high Ta (~37 °C). Surprisingly, heat gain was higher at low Ta (0.30W) than at high Ta (0.17W) due in part to an 18% reduction in hovering metabolic rate. Total evaporative heat dissipation did not change across temperatures, but the contribution of respiratory evaporation decreased from 30 to 8% at higher temperature suggesting a restriction in respiratory evaporation. Because birds began panting while hovering at 36 °C it is likely that hovering at higher temperature was more challenging, and that the lower heat load predicted by our model might be due to under estimation of radiant heat gain. Heat stress during hovering at temperatures above 36 °C is consistent with our observations that hummingbirds in the field begin behaviorally regulating their surface temperature at ~38 °C.
