Energetics and Thermoregulation by Small Passerines of the Humid, Lowland Tropics

Abstract

Birds of the open, humid lowland tropics encounter challenging thermal con- ditions-high temperatures, high humidity, and intense solar radiation. I examined how one such species, the Variable Seedeater (Sporophila aurita, Emberizidae), responds to tempera- ture by measuring its body temperature (Tb), metabolic heat production (Hm calculated from oxygen consumption), and evaporative heat loss (HIf calculated from evaporative water loss) at stable air temperatures (Ta) between 14 and 460C. I also measured basal metabolic rate (BMR) and Tb of the Variable Seedeater's diminutive (7.5 g) congener, the Ruddy-breasted Seedeater (S. minuta). All measurements utilized fasted, active-phase birds that were resting in the dark. BMR was lower than expected allometrically in both species, averaging 76% of predicted (or 0.718 kJ/h) in the 9.8-g Variable Seedeater (n = 11) and 67% of predicted (or 0.525 kJ/h) in the Ruddy-breasted Seedeater (n = 3). The Variable Seedeater's thermoneutral zone (TNZ) was relatively high (28.9 to 39.2'C). Below the TNZ, Hm was linearly related to Ta as follows: Hm (kJ/h) = 2.22 - 0.052 Ta. Thermal conductance, as indicated by the slope of this relation, was 12% lower than predicted allometrically. Data for 14 tropical bird species show BMR and thermal conductance to be linked; species with relatively high thermal con- ductance have a high BMR and vice versa. Evaporative cooling is relatively ineffective in the humid tropics, and compared with most birds Variable Seedeaters have a blunted evapo- rative response to heat. They dissipate evaporatively a maximum of 127% of their metabolic heat production at high Ta, even when measured in air only half as humid as that of their native habitat. Consequently, Variable Seedeaters employ hyperthermia (elevated Tb ) to cope with heat. They are more tolerant of hyperthermia than most bird species and survive Tbs that are among the highest recorded for birds (46.8 to 47.0?C). Tolerance of hyperthermia is advantageous because it allows Variable Seedeaters to maintain an unusually large Tb-T, gradient in hot environments (0.8 to 1.4'C at Ta = 43?C), and thereby to dissipate heat pas- sively. Variable Seedeaters are able to circumvent partially the well-known temperature de- pendency of chemical reactions (i.e. the Arrhenius-van't Hoff effect). This enables them to become progressively hyperthermic at Tas above 35'C with relatively little increase in met- abolic heat production, their Hm increasing at only 52% of the allometrically predicted rate above the TNZ. Variable Seedeaters possess several traits that enhance their tolerance of high Ta, yet because of their small size and limited thermal inertia their principal response to heat stress in the field is to avoid it behaviorally rather than to overcome it physiologically.