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.

依据捕食规避假说（predation avoidance hypothesis）与觅食效率假说（foraging efficiency hypothesis），混合鸟群（mixed flocks）中的鸟类能够提升觅食效率，或可投入更多时间进食、减少警戒天敌的时长。上述假说均预测，混合鸟群中的鸟类可获得生存收益。因此，组建混合鸟群可作为应对鸟类所面临严苛环境的策略——这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“加入鸟群可使群体成员获得收益”这一假说，以及与之相关的预测：鸟类会利用这些收益优势，在季节间及季节内的寒冷干燥天气下更频繁地结群，以应对此类环境压力。我们在阿根廷亚热带永加斯山麓开展调查，记录混合鸟群的存在情况、结群倾向（flocking propensity）、混合鸟群中的物种数量与个体数量，以此探究鸟类结群行为的季节节律，以及它们对温度和湿度这两项气候变量的响应。研究结果显示，相较于气候更为温和的季节，在寒冷干燥的季节，鸟类的结群倾向更高，混合鸟群出现的频次也更多；且在季节内，较低的气温会触发结群行为。尽管我们预先推测季节间会存在结群差异，但研究结果同时显示，鸟类还会对季节内气温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论断。尽管结群所带来的收益尚未明确，但无论其具体为何，都应结合鸟类生活史特征（life-history traits）的季节变异来加以理解。