Interactions between humidity and evaporative heat dissipation in a passerine bird

Environmental humidity is thought to be a major determinant of evaporative cooling capacity at high air temperatures (T_a), but the technical challenges of experimentally manipulating humidity in respirometry chambers have resulted in far less being known about the effects of humidity compared to those of T_a. We tested the prediction that at T_a approaching and exceeding normothermic body temperature (T_b), high humidity would result in higher T_b, lower evaporative water loss (EWL) and/or higher resting metabolic rate (RMR) in a passerine bird, the white-browed sparrow-weaver (Plocepasser mahali). We used open-system flow-through respirometry to measure EWL, RMR and T_b in sparrow-weavers experiencing T_a = 36-44 °C and chamber humidities of 6, 13, 19 or 25 g m^- 3. Increasing humidity was associated with significantly higher T_b. The strongest effect of humidity, however, involved significant increases in RMR; at T_a = 40 °C, RMR at a humidity of 25 g m^- 3 was ~ 40% higher compared to a humidity of 6 g m^- 3. Moreover, the interaction between T_a and humidity exerted a significant effect on the ratio of evaporative heat loss (EHL) to metabolic heat production (MHP), evident as an increasing effect of humidity with increasing T_a. Our results, when compared with those of the limited number of previous studies that involved similar ranges of T_a and humidity, reveal that the relative effects of humidity on EWL and RMR vary among avian taxa, and support the notion that the overall effect of high humidity is a reduction in maximum EHL/MHP.