Seasonal variation in great tit energy requirements: reallocation versus increased demand

Understanding how birds annually allocate energy to cope with changing environmental conditions and physiological states is a fundamental question in avian ecology. The two main hypotheses to explain annual patterns in energy use are "reallocation" and "increased demand". The reallocation hypothesis suggests equal energetic costs in winter and breeding seasons, while the increased demand suggests that energy demand should be highest during breeding. Under the standard aerobic capacity model of endothermy, birds are expected to 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 reallocation versus increased demands in the energy requirements of a small, 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 late winter and during the chick-rearing period (breeding). We also assessed whether, and to what extent, metabolic rates conform to the predictions of the aerobic capacity model of endothermy. We found that great tits showed no substantial differences in energy expenditure between winter and the breeding season, providing support for the reallocation hypothesis. Only mass-independent Msum showed seasonal variation, with significantly higher values (~4%) in winter compared to the breeding season. Our results also lend support to the predictions of the aerobic capacity model for the evolution of endothermy, as we found that whole-body BMR and Msum were positively related. We argue that both energy reallocation and the limited increase in mass-independent Msum are consistent with the relatively mild winter temperatures recorded during our study period. Our results confirm that both BMR and Msum are flexible traits that vary in ways that are consistent with expected cold- and/or activity-induced costs.阐明鸟类如何在年度周期内分配能量以应对动态变化的环境条件与生理状态，是鸟类生态学（avian ecology）领域的核心科学问题之一。解释能量使用年度模式的两大主流假说分别为能量重分配假说（reallocation hypothesis）与需求增加假说（increased demand hypothesis）：能量重分配假说认为，鸟类在冬季与繁殖季的能量消耗水平相当；而需求增加假说则提出，繁殖期的能量需求应为全年最高。在经典的恒温动物有氧能力模型（aerobic capacity model of endothermy）框架下，鸟类应根据低温或活动引发的能量消耗预期，调整自身身体质量和/或代谢强度。本研究以温带地区小型留居雀形目鸟类大山雀（great tit, *Parus major*）为研究对象，旨在从其能量需求中挖掘与上述两大假说相关的代谢特征。为此，我们分别在晚冬以及雏鸟育雏期（繁殖季）测定了个体的整体代谢率与体质量校正后的代谢率，包括基础代谢率（basal metabolic rate, BMR）、顶峰代谢率（summit metabolic rate, Msum）与野外代谢率（field metabolic rate, FMR）。此外，我们还评估了代谢率是否符合、以及在多大程度上符合恒温动物有氧能力模型的预测。研究结果显示，大山雀在冬季与繁殖季的能量消耗并无显著差异，为能量重分配假说提供了直接支持。仅体质量校正后的顶峰代谢率（mass-independent Msum）存在季节差异：冬季的该指标较繁殖季显著升高约4%。此外，我们发现整体基础代谢率与整体顶峰代谢率呈显著正相关，这也为恒温动物有氧能力模型的进化预测提供了支撑。我们认为，能量重分配现象以及体质量校正后的顶峰代谢率的小幅升高，均与本研究期间记录到的温和冬季气温相契合。本研究结果证实，基础代谢率与顶峰代谢率均为可塑性性状，其变化模式与低温或活动引发的能量消耗预期相符。