The role of the plumage in heat transfer processes of birds

The plumage of birds provides a critical thermal buffer between the animal and its environment. Rates of energy expenditure are strongly influenced by the thermal properties of the environment or the microclimates the animal occupies. Current data suggest that the addition of solar radiation is equivalent to three to four-fold changes in wind speed and that solar heat gain can be extremely sensitive to changes in wind speed. Dry heat transfer through the plumage occurs by three avenues 1) conduction and free convection through air 2) conduction along the solid elements of the plumage and 3) radiation. Overall, about 95% of the total heat flow is evenly divided between the first two avenues. Radiative heat transfer accounts for only about 5% of total heat flow, Plumage color, as well as the microstructure and micro-optical properties of plumage elements, when combined with environmental properties (e.g., wind speed), determine the radiative heat loads that birds acquire from solar radiation. Although plumage color or reflectivity determines the fraction of incident solar radiation that is absorbed by the plumage and generates heat, the fraction of this heat that contributes to the thermal load on the animal can vary greatly. In a fibrous coat such as a plumage, there is some variable penetration into the coat, with absorption over a range of coat depths. Factors such as feather microoptics and structure are critical determinants of radiation penetration into avian coats. Significant differences in solar heat loads can also result from behavioral adjustments in plumage thickness.