The ugly duckling: a thermal viewpoint

On theoretical grounds it was understood since long that birds with a white plumage may experience a higher heat load in the sun than black birds, when the white plumage is penetrable for solar radiation and the black plumage not. Experimental evidence was scarce and not readily understood, however. It is argued that the core temperature of a model with an insulating cover can give a good estimate of the operative temperature for a bird or mammal when the reflectivity and thermal properties of the coat are the same for both. The temperature in the core of an aluminium bulb covered with the white pelt of a swan nestling was always higher in the sun than that in a smooth black aluminium bulb. The difference was maximal in moderately windy conditions where the temperature excess for the black bulb was 10degreesC and 16degreesC for the pelt-covered model. Experiments with artificial pelts gave far stronger effects but it seems plausible that natural fur or plumage can be equally effective. A simple treatment of the basic mechanism and the quantitative aspects of the phenomenon is given. (C) 2002 Elsevier Science Ltd. All rights reserved.