Body Surface Temperatures and Thermoregulation in the Black-Capped Chickadee (Parus atricapillus)

Surface temperature (Ts) on most parts of the body of the black-capped chickadee is linearly related to ambient temperature (Ta) between 27 and -22 C. The slope and intercept of regressions of Ts on Ta are generally related as predicted by a model developed previously. The head typically has the highest Ts of all body parts and accordingly is expected to exhibit the highest rate of heat loss per unit area by convection and radiation at given Ta. The breast is warmer than the side of the torso, the beak is kept at relatively high Ts, and Ts on the feet and legs is strongly dependent on whether the appendages have recently been ensconced in the ventral plumage. Differences in Ts are remarkably small between the chickadee and raven, which represent virtual extremes of body size within the Passeriformes. Metabolic rates of chickadees can be calculated from Ts with reasonable success given inadequacies in present knowledge of some relevant parameters. Chickadees show large increases in ptiloerection as Ta is lowered in the range of Ta well below thermoneutrality; significant consequent changes in plumage conductance, body convection coefficient, and operative body surface area are possible and require appraisal in regard to heattransfer analysis. Subcutaneous temperature may fall significantly with Ta on the back and rump but is consistently high on the breast. On the breast the plumage presents most of the resistance to heat flow from bird to environment, and the resistance posed by the living tissues between body core and skin is minor. Plumage and tissue conductances are quantitatively estimated using a new approach based on core temperature, subcutaneous temperature, and Ts; these estimates are extensively compared with others derived from the literature. Apparently, increases in ptiloerection are accompanied by decreases in plumage conductance but increases in plumage conductivity. The first spectral emittance data to appear in the journal literature for plumage of a bird species at wavelengths longer than 2.5 μm are presented.