Basal metabolic rate and energetic cost of thermoregulation among migratory and resident blue tits

Metabolic rates may be informative of adaptations to migration or wintering at high latitudes and may therefore be particularly interesting in partial migrants requiring adaptations to both migration and residency. To study the extent of physiological adaptations in migratory and resident blue tits Cyanistes caeruleus during the period of autumn migration in southern Sweden, we measured basal metabolic rate (BMR) and cost of thermoregulation at 0°C (CTR0). In contrast to other migrants en route, migratory blue tits had lower BMR than residents. As migratory blue tits travel extraordinarily slowly on autumn migration and residents suffer from harsher climate and severe competition, residents may need dynamic adjustments that involve larger metabolic costs than migrants. Resident males had lower CTR0 than migrants and resident females. Resident males are socially dominant, which suggests that they have priority of access to food sources during summer and early autumn. They also spend more time on moult, which would give them the time and energy needed for molting into a plumage of higher insulation quality than is possible for migrants and resident females.