Effect of temperature and humidity on evaporative water loss in Anna's hummingbird (Calypte anna)

Evaporative water loss (EWL), oxygen concumption $$\dot V_{O_2 } $$, and body temperature (Tb) of Anna's Hummingbirds (Calypte anna; ca. 4.5g) were measured at combinations of ambient temperature (Ta) and water vapor density (?va) ranging from 20 to 37 °C and 2 to 27 g·m-3, respectively. The EWL decreased linearly with increasing ?va at all temperatures. The slopes of least squares regression lines relating EWL to ?va at different temperatures were not significantly different and averaged-0.50 mg H2O·m-3·g-2·h-1 (range:-0.39 to-0.61). Increased ?va restricted EWL in C. anna more than has been reported for other endotherms in dry air. The percent of metabolic heat production dissipated by evaporation ($$(\dot H_e /\dot H_m )$$) was lower than that of other birds in dry air, but higher than that for other birds at high humidity when Ta 33 °C. When Ta>33 °C the effect of humidity on $$(\dot H_e /\dot H_m )$$ was similar to that in other birds. Calypte anna might become slightly hyperthermic at Ta>37 °C, which could augment heat transfer by increasing the Tb-Ta gradient. Body temperature for C. anna in this study was 43 °C (intramuscular) at Tas between 25 and 35 °C, which is above average for birds. It is estimated that field EWL is less than 30% of daily water loss in C. anna under mild temperature conditions (<35 °C).