THE ENERGETIC COST OF FEATHER SYNTHESIS IS PROPORTIONAL TO BASAL METABOLIC-RATE

The cost of feather production, C(f) (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 differences 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 efficiency 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, BMR(m) (kJ . g-1 . d-), where BMR(m) currently showed the highest degree of explanation, although other factors that influence BMR(m) cannot be discounted. The C(f) for a species with known BMR(m) may be estimated from the equation C(f) = 270 BMR(m). Species with a relatively high BMR for their size also have a relatively high C(f). The tight association of C(f) and BMR(m) between species, and the low efficiency 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.