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

Summary

Evaporative water loss (EWL), oxygen concumption $\dot V_{O_2 }$, and body temperature (T_b) of Anna's Hummingbirds (Calypte anna; ca. 4.5g) were measured at combinations of ambient temperature (T_a) and water vapor density (ϱ_va) ranging from 20 to 37 °C and 2 to 27 g·m^-3, respectively. The EWL decreased linearly with increasing ϱ_va at all temperatures. The slopes of least squares regression lines relating EWL to ϱ_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 ϱ_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 T_a 33 °C. When T_a>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 T_a>37 °C, which could augment heat transfer by increasing the T_b-T_a gradient. Body temperature for C. anna in this study was 43 °C (intramuscular) at T_as 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).