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 Mu ller) exposed to ambient temperatures (T-a) and convective conditions typic al of that which birds experience in nature while perched in the open or fo raging on the ground. We addressed the hypothesis that resting and active b irds exposed to similar net convective conditions will exhibit similar surf ace temperatures (T-s) and metabolic energy expenditures. To test this hypo thesis, resting birds were exposed to a wind speed equivalent to the treadm ill speed (0.5 m s(-1)) for a hopping bird (active). T-s of resting birds i n no wind, resting birds exposed to wind, and active birds were measured wi th infrared thermography at T-a between 0 degrees and 25 degrees C. Metabol ic heat production was estimated from measures of respiratory gases at T-a between -5 degrees and 25 degrees C. For resting birds in no wind, resting birds in wind, and active birds, T-s decreased with decreasing T-a. The eff ects of variation in T-a on T-s depended on activity level (F=3.91, df=2,40 , P=0.0280). The regression relationship of T-s on T-a, however, did not di ffer 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 highe r 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 increase d with decreasing T-a. 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) expose d 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 evid ence that exercise generated compensates for thermostatic requirements at T -a just below thermoneutrality, which resembles conditions under which hous e finches naturally forage. We conclude that the compensation of exercise-g enerated heat for regulatory thermogenesis may occur more frequently under natural environmental conditions than implied by most previous investigator s and can result in considerable energy savings for birds living in cold en vironments.