Data from: Bird mixed-species flock formation is driven by low temperatures between and within seasons in a Subtropical Andean-foothill forest

According to both the predation avoidance and foraging efficiency hypotheses, birds within mixed flocks increase their foraging efficiency and/or can spend more time feeding and less time looking out for predators. These hypotheses predict that birds in mixed flocks obtain benefits. Thus, mixed flock formation could serve as a strategy to cope with difficult conditions imposed on birds such as climatic conditions that ultimately result in a change in predation pressure or food resources. We evaluate the hypotheses that forming part of a flock confers benefits to its members and the associated prediction that birds will take advantage of these benefits and flock more often under cold and dry weather conditions between and within seasons to cope with such conditions. We surveyed the presence of mixed flocks, flocking propensity, number of species and individuals in mixed flocks in the subtropical Yungas-foothill of Argentina, to examine seasonality, flocking behavior of birds and their responses to two climatic variables: temperature and humidity. Bird species presented a higher flocking propensity and mixed flocks occurred more frequently during the dry and cold seasons than during the more benign seasons, and lower values of temperature within seasons triggered the flocking behavior. Although effects between seasons were expected, birds also showed a short-term response to small changes in temperature within seasons. These results strengthen the ideas proposed by the foraging hypothesis. Although benefits derived from flocking have yet to be determined, whatever they are should be understood in the context of seasonal variation in life-history traits.

基于反捕食假说（predation avoidance hypothesis）与觅食效率假说（foraging efficiency hypothesis），混合鸟群中的鸟类可提升自身觅食效率，或能投入更多时间进食、更少时间用于警戒天敌。上述假说均预测，混合鸟群中的鸟类可获得生存收益。因此，组建混合鸟群或可作为应对鸟类所面临恶劣环境的策略——此类环境包括最终改变捕食压力或食物资源的气候条件。本研究旨在验证两大核心假说：其一，加入鸟群可为个体带来生存收益；其二，鸟类会利用此类收益，在季间与季内的寒冷干燥天气中更频繁地结群，以应对此类恶劣环境。我们在阿根廷亚热带永加斯山麓（Yungas-foothill）区域开展调查，记录混合鸟群的存在情况、结群倾向、混合鸟群中的物种数与个体数，以此探究鸟类的季节性规律、结群行为，以及它们对温度与湿度这两类气候变量的响应。研究结果显示，相较于气候更温和的季节，寒冷干燥季节的鸟类结群倾向更高，混合鸟群出现频次也更多；且季内温度较低时会触发结群行为。尽管我们预期季间会存在此类效应，但研究同时发现，季内鸟类也会对温度的小幅波动产生短期响应。上述结果进一步支撑了觅食效率假说的相关观点。尽管结群所带来的具体收益仍有待明确，但无论此类收益为何，均需结合生活史特征的季节变异背景进行解读。