Energetic Features of Avian Thermoregulatory Responses

Abstract

Birds are found on every continent and in or above the surrounding seas. With the wide distribution of the group, many of its members encounter challenging thermal conditions, even though migratory movements afford certain species opportunities for evading winter or summer extremes. The thermoregulatory processes that contribute to birds’ maintaining homeostasis in the face of these conditions have received considerable attention (see, e.g., Calder and King 1974; Dawson 1984; Whittow 1986; Marsh and Dawson 1989). We shall explore a different pathway in this review, emphasizing the energetic costs of these processes, that is, the costs of heat and cold defense in normothermic birds. These activities are complicated for many species by small size. The vast majority of birds weigh less than 100 g and many are smaller than 10 g. Such masses involve low thermal inertia and narrow zones of thermal neutrality (cf. Calder and King 1974, pp. 278-279). In the ensuing discussion, we shall strive to integrate laboratory and field observations to make our considerations as ecologically relevant as possible