Evaporative water loss (EWL), oxygen concumption (V(o)2), and body temperature (T(b)) of Anna's Hummingbirds (Calypte anna; ca. 4.5 g) were measured at combinations of ambient temperature (T(a)) and water vapor density (rho(va)) ranging from 20 to 37-degrees-C and 2 to 27 g. m-3, respectively. The EWL decreased linearly with increasing rho(va) at all temperatures. The slopes of least squares regression lines relating EWL to rho(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 rho(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 (H(e)/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-degrees-C. When T(a) > 33-degrees-C the effect of humidity on H(e)/H(m) was similar to that in other birds. Calypte anna might become slightly hyperthermic at T(a) > 37-degrees-C, which could augment heat transfer by increasing the T(b) - T(a) gradient. Body temperature for C. anna in this study was 43-degrees-C (intramuscular) at T(a)S between 25 and 35-degrees-C, which is above average for birds. It is estimated that field EWI, is less than 30% of daily water loss in C. anna under mild temperature conditions (< 35-degrees-C).
POWERS, DR
EFFECT OF TEMPERATURE AND HUMIDITY ON EVAPORATIVE WATER-LOSS IN ANNAS HUMMINGBIRD (CALYPTE-ANNA)
Journal Of Comparative Physiology B-Biochemical Systems And Environmental Physiology