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

The cost of feather production, C(f) (kJ . [g dry feathers]-1), differ s substantially between species. We studied the molt cost in one insec tivorous songbird (bluethroat, Luscinia s. svecica) and one granivorou s songbird (common redpoll, Carduelis f. flammea), We wanted to test w hether differences in diet, body mass (or basal metabolic rate, BMR), or the latitude of molt could explain interspecific differences. In ea ch individual, the resting metabolism, as measured by indirect calorim etry, was positively correlated with feather production rate. The cost Of feather synthesis was estimated at 836 and 683 kJ . (g dry feather s)-1 in the bluethroats and redpolls, respectively. The efficiency of feather production was 2.6% and 3.1%. It was concluded that neither di et nor latitudinal constraints alone could explain the differences fou nd between species. The cost of feather production was significantly c orrelated 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. T he C(f) for a species with known BMR(m) may be estimated from the equa tion C(f) = 270 BMR(m). Species with a relatively high BMR for their s ize also have a relatively high C(f). The tight association of C(f) an d BMR(m) between species, and the low efficiency values of feather syn thesis, suggests that feather production costs include more than the c osts for keratin synthesis: they mainly consist of costs of maintainin g tissues necessary for feather production.