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.

依据反捕食假说（predation avoidance hypothesis）与觅食效率假说（foraging efficiency hypothesis），混合鸟群（mixed flocks）内的鸟类可提升觅食效率，或投入更多时间取食、减少警戒天敌的时长。上述两大假说均预测，混合鸟群中的鸟类能够获得生存收益。因此，混合鸟群的形成或可作为鸟类应对严苛生存环境的策略——此类环境涵盖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益”这一假说，以及与之相关的预测：鸟类会利用此类收益，在季节间及季节内的寒冷干燥天气中更频繁结群，以应对这类环境。本研究对阿根廷亚热带永加斯山麓区域的混合鸟群存在情况、结群倾向（flocking propensity）、混合鸟群内的物种数与个体数开展了调查，以探究鸟类的季节性特征、结群行为，及其对气温、湿度两类气候变量的响应。研究结果表明，相较于环境更为温和的季节，干旱寒冷季节中鸟类的结群倾向更高，混合鸟群的出现频率也显著提升；且季节内的低温会诱发结群行为。尽管我们曾预期季节间会存在效应差异，但鸟类同样会对季节内气温的小幅变化产生短期响应。上述结果进一步支撑了觅食效率假说的相关论断。尽管结群带来的具体收益仍有待探明，但无论该收益为何，均需结合鸟类生活史特征的季节变异进行解读。