The influence of wind and locomotor activity on surface temperature and energy expenditure of the Eastern house finch (Carpodacus mexicanus) during cold stress

We investigated the extent to which exercise-generated heat compensates for regulatory thermogenesis of Eastern house finches (Carpodacus mexicanus Müller) exposed to ambient temperatures (Ta) and convective conditions typical of that which birds experience in nature while perched in the open or foraging on the ground. We addressed the hypothesis that resting and active birds exposed to similar net convective conditions will exhibit similar surface temperatures (Ts) and metabolic energy expenditures. To test this hypothesis, resting birds were exposed to a wind speed equivalent to the treadmill speed (0.5 m s-1) for a hopping bird (active). Ts of resting birds in no wind, resting birds exposed to wind, and active birds were measured with infrared thermography at Ta between 0 degrees and 25 degrees C. Metabolic heat production was estimated from measures of respiratory gases at Ta between -5 degrees and 25 degrees C. For resting birds in no wind, resting birds in wind, and active birds, Ts decreased with decreasing Ta. The effects of variation in Ta on Ts depended on activity level (F=3.91, df=2,40, P=0.0280). The regression relationship of Ts on Ta, however, did not differ significantly between resting birds exposed to wind and active birds (F=0.12, df=2,40, P=0.8865), whereas the slope was lower and intercept higher for resting birds in no wind compared with those of resting birds exposed to wind and active birds combined (F=20.96, df=2,42, P<0.0001). Metabolic heat production for resting birds exposed to wind and active birds increased with decreasing Ta. Average metabolic heat production of resting (46.01 mW g-1+/-10.60 SD) and active birds (47.63 mW g-1+/-8.76 SD) exposed to similar net convective conditions did not differ significantly (F=3.87, df=1,44, P=0.0556). These results support our hypothesis and provide evidence that exercise generated compensates for thermostatic requirements at Ta just below thermoneutrality, which resembles conditions under which house finches naturally forage. We conclude that the compensation of exercise-generated heat for regulatory thermogenesis may occur more frequently under natural environmental conditions than implied by most previous investigators and can result in considerable energy savings for birds living in cold environments.