Thermoenergetics of pre-moulting and moulting kookaburras (Dacelo novaeguineae): they're laughing

We examined the effect of temperature on resting metabolic rate in seven field-captured laughing kookaburras (Dacelo novaeguineae) during late winter and early spring. Basal metabolic rate averaged 201 ± 3.4 ml O2 h-1 (0.603 ml O2 g-1 h-1). Overall thermal conductance (Ko) declined with ambient temperature (Ta) and averaged 0.026 ml O2 g-1 h-1°C-1 at Tas ≤ 10°C. Day-night differences in body temperatures (2.6°C) and in alpha-phase versus rho-phase minimum metabolic rates were much greater (33%) than predicted for 340-g nonpasserine birds and suggest that these animals operate as low-metabolic intensity animals in their rest phase, but normal-metabolic intensity animals during their active phase. Metabolic rate was measured in four of the same birds undergoing moult. Thermal conductance increased to 60% above pre-moult values about 6 weeks after moult began. Basal metabolic rate of moulting birds showing peak thermal conductance readings averaged 17 ml O2 h-1 higher than pre-moult measurements. Although this increase was not statistically significant, we believe the moult costs of kookaburras are too low to overcome the inherent variability of BMR determination. We suggest that moult costs of kookaburras are only somewhat higher than the measured costs of protein synthesis of other endotherms.