Energetics and thermoregulation of the Gouldian Finch (Erythrura gouldiae)

Abstract

We examined the physiological responses of captive-bred Gouldian Finches (Erythrura gouldiae) to temperature by measuring their metabolic heat production (H_m, calculated from oxygen consumption), body temperature (T_b), and evaporative heat loss (H_e, calculated from evaporative water loss) at ambient temperatures (T_a) between 0 and 44°C. Overall, we found the Gouldian Finch to resemble other small tropical seed-eaters that forage in open, sunlit situations. Its active-phase basal metabolic rate (13.4 mW g^–1; n = 11) was relatively low (61% of predicted allometrically) and it had only a modest capacity for evaporative cooling at high temperatures, dissipating evaporatively a maximum of 118% of its metabolic heat production above 40°C. Like other small tropical birds, the Gouldian Finch develops substantial hyperthermia at high temperature, maintaining its T_b an average of 1.4°C above T_a at T_as between 41.8 and 43.6°C. At T_as above the upper critical temperature (38°C), metabolic heat production increases linearly with T_a according to the relation: H_m (mW g^–1) = –12.4 + 0.68 T_a. The slope of this relation, termed the coefficient of heat strain (h_s), provides an integrated measure of the metabolic response to heat. For 38 bird species, h_s (mW g^–1 °C^–1) varies with body size according to the relation: log(h_s) = log(7.83) – 0.54log(m), where m equals mass in grams. The Gouldian Finch's h_s is only 40% of that predicted for a 17.1-g bird by this equation, proportionately the lowest value yet measured. Although aviculturalists widely regard the Gouldian Finch as cold intolerant, T_a in its northern Australian range sometimes falls to below freezing. Not surprisingly, we found that the  Gouldian Finch is actually better insulated than the average songbird its size, its thermal conductance (0.184 mL O₂ g^–1 h^–1 °C^–1) being 81% of that predicted allometrically.