ИСТИНА

Understanding how birds annually allocate energy to cope with changing environmental conditions and physiological states is a crucial question in avian ecology. There are several hypotheses to explain species' energy allocation. One prominent hypothesis suggests higher energy expenditure in winter due to increased thermoregulatory costs. The "reallocation" hypothesis suggests no net difference in seasonal energy requirements, while the "increased demand" hypothesis predicts higher energy requirements during the breeding season. Under the aerobic capacity model of endothermy, birds adjust the mass and/or metabolic intensity of their bodies in ways that are consistent with expected cold- and/or activity-induced costs. Here, we look for metabolic signatures of seasonal variation in energy requirements of a resident passerine of a temperate-zone (great tit, Parus major). To do so, we measured whole-body and mass-independent basal (BMR), summit (Msum), and field (FMR) metabolic rates during winter and during breeding in Belgian great tits. We also assessed whether, and to what extent, metabolic rates conformed to the predictions of the aerobic capacity model. Our result did not support the aerobic capacity model. Breeding season showed a 10% higher whole-body BMR and FMR compared to winter, while Msum had a 7% decline from winter to breeding. Mass-independent metabolic rates showed a 10% increase in BMR and a 7% decrease in Msum from winter to breeding. We argue that these modest seasonal changes are consistent with the relatively mild winter temperatures recorded during our study period and that the reallocation hypothesis is more closely aligned with our findings.