MOLT AND BASAL METABOLIC COSTS IN MALES OF 2 SUBSPECIES OF STONECHATS- THE EUROPEAN SAXICOLA-TORQUATA-RUBICULA AND THE EAST-AFRICAN SAXICOLA-TORQUATA-AXILLARIS

The circannual patterns in resting metabolic rate (RMR) of males of tw o subspecies of stonechats, the European Saxicola torquata rubicula an d the East African S. t. axillaris, are compared. As the birds from th e two subspecies were raised and kept under comparable laboratory cond itions, differences in metabolic rate between the two subspecies had t o be genetically determined. RMR peaked during moult in both subspecie s. During the rest of the year RMR was fairly constant in both subspec ies and assumed to reflect basal metabolic rate (BMR). African stonech ats had a 22% lower mass specific BMR than European stonechats, which is thought to reflect a genetical physiological adaptation to the diff erences in environmental circumstances they experience in the field. A low BMR makes an animal more susceptible to cold. Hence, the relative ly high plumage mass in the African compared to the European stonechat may be functionally linked to its relatively low BMR. Moult costs, ca lculated from the plumage masses and the differences in RMR inside and outside the moult period, tended to be higher in the European compare d to the African stonechats. These data and an interspecific compariso n of moult costs over various species of birds support the earlier not ion by Lindstrom et al. (1993) that moult costs are more closely linke d with BMR than with body mass or rate of moult. The relation between moult costs and BMR and the fact that the efficiency of moult is extre mely low (3.8 and 6.4% for European and African stonechats, respective ly) suggest that the maintenance of specific tissues necessary for mou lt is a large cost factor. Alternatively, impaired insulation during m oult may necessitate an increased metabolic capacity which may be asso ciated with an increased RMR.