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The cost of feather production, Cf (kJ · [g dry feathers]⁻¹), differs substantially between species. We studied the molt cost in one insectivorous songbird (bluethroat, Luscinia s. svecica) and one granivorous songbird (common redpoll, Carduelis f. flammea). We wanted to test whether diferences in diet, body mass (or basal metabolic rate, BMR), or the latitude of molt could explain interspecific differences. In each individual, the resting metabolism, as measured by indirect calorimetry, was positively correlated with feather production rate. The cost of feather synthesis was estimated at 836 and 683 kJ · (g dry feathers)⁻¹ in the bluethroats and redpolls, respectively. The efciency of feather production was 2.6% and 3.1%. It was concluded that neither diet nor latitudinal constraints alone could explain the differences found between species. The cost of feather production was significantly correlated with both body mass and mass-specific BMR, BMRm (kJ · g⁻¹ · d⁻¹), where BMRm currently showed the highest degree of explanation, although other factors that influence BMRm cannot be discounted. The Cf for a species with known BMRm may be estimated from the equation Cf = 270 BMRm. Species with a relatively high BMR for their size also have a relatively high Cf. The tight association of Cf and BMRm between species, and the low efciency values of feather synthesis, suggests that feather production costs include more than the costs for keratin synthesis: they mainly consist of costs of maintaining tissues necessary for feather production.

The Energetic Cost of Feather Synthesis Is Proportional to Basal Metabolic Rate