Data from: To eat and not be eaten: diurnal mass gain and foraging strategies in wintering great tits

Adaptive theory predicts that the fundamental trade-off between starvation and predation risk shapes diurnal patterns in foraging activity and mass gain in wintering passerine birds. Foragers mitigating both types of risk should exhibit a bimodal distribution (increased foraging and mass gain early and late in the day), whereas both foraging and mass gains early (vs. late) during the day are expected when the risk of starvation (vs. predation) is greatest. Finally, relatively constant rates of foraging and mass gain should occur when the starvation-predation risk trade-off is independent of body mass. Using automated feeders with integrated digital balances, we estimated diurnal patterns in foraging and body mass gain to test which ecological scenario was best supported in wintering great tits Parus major. Based on data of 40 consecutive winter days recording >12 000 body masses of 28 individuals, we concluded that birds foraged and gained mass early during the day, as predicted by theory when the starvation-predation risk trade-off is mass-dependent and starvation risk outweighs predation risk. Slower explorers visited the feeders more often, and decreased their activity along the day more strongly, compared to faster explorers, thereby explaining a major portion of the individual differences in diurnal patterning of foraging activity detected using random regression analyses. Birds did not differ in body mass gain trajectories, implying both that individuals differed in the usage of feeders, and that unbiased conclusions regarding how birds resolve starvation-predation risk trade-off require the simultaneous recording of foraging activity and body mass gain trajectories. Our study thereby provides the first unambiguous demonstration that individual birds are capable of adjusting their diurnal foraging and mass gain trajectories in response to ecological predictors of starvation risk as predicted by starvation-predation risk trade-off theory.

适应性理论预测，饥饿与捕食风险间的核心权衡塑造了越冬雀形目鸟类觅食活动与体重增加的昼夜节律模式。若觅食者需同时规避这两类风险，则其行为应呈现双峰分布（清晨与傍晚时段觅食活动与体重增加水平提升）；当饥饿风险（相较于捕食风险）在当日早期更为突出时，理论预期鸟类会在当日早期提升觅食与体重增长；而当捕食风险（相较于饥饿风险）主导当日晚期时，则应在当日晚期进行更多觅食与体重增长。最后，当饥饿-捕食风险权衡（starvation-predation risk trade-off）与个体体重无关时，鸟类的觅食与体重增长速率应相对恒定。
本研究借助集成数字天平的自动喂食器，对越冬大山雀（Parus major）的觅食与体重增加昼夜模式进行量化估算，以验证哪种生态情景最符合实际观测结果。本研究基于连续40个冬季日的观测数据——涵盖28只个体的逾12000次体重记录——得出结论：鸟类在当日早期开展觅食并增加体重，这与"饥饿-捕食风险权衡依赖于个体体重且饥饿风险占优"的理论预测相符。
相较于探索速度更快的个体，探索速度较慢的大山雀造访喂食器的频率更高，且全天活动强度的下降幅度也更为显著，这解释了通过随机回归分析（random regression analyses）检测到的觅食活动昼夜模式的大部分个体差异。
不同个体的体重增加轨迹并无显著差异，这既表明个体间对喂食器的使用存在差异，也意味着若要客观探究鸟类如何应对饥饿-捕食风险权衡，需同时记录觅食活动与体重增加轨迹。
本研究由此首次明确证实，单个鸟类能够依据饥饿风险的生态预测因子，调整其昼夜觅食与体重增加轨迹，这与饥饿-捕食风险权衡理论的预测一致。