Energy Requirements for Molt in the KestrelFalco tinnunculus

We estimated energy requirements for plumage replacement in the kestrel (Falco tinnunculus) by comparing O₂, consumption V̇o2 and metabolizable energy intake during molt and nonmolt. The energy expenditure for feather synthesis (S) as derived from the regression of basal metabolic rate (BMR) on molt intensity was 104 kJ·(g dry feathers)⁻¹. As derived from the regression of the resting metabolic rate of nonfasting birds at night (RMR) on molt intensity, S was 108 kJ·g⁻¹, and statistically indistinguishable from the BMR estimate. During the first part of molt the coefficient of temperature dependence of the standard metabolic rate below thermoneutrality (SMR) increased from 0.050 to 0.070 W·° C⁻¹. The metabolizable energy required for maintenance (Mm) increased with increasing body mass. The Mm in males was higher than in females of the same body mass. The Mm during molt was higher than during nonmolt episodes. The estimate for the cost of feather synthesis based on this difference was S = 117 kJ·g⁻¹. The cost of feather synthesis in the kestrel is thus much lower than values reported for three passerines (230-835 kJ·g⁻¹). This difference in efficiency may be related to diet (carnivorous vs. granivorous birds) or body mass.