Data from: Smaller beaks for colder winters: Thermoregulation drives beak size evolution in Australasian songbirds

Birds’ beaks play a key role in foraging, and most research on their size and shape has focused on this function. Recent findings suggest that beaks may also be important for thermoregulation, and this may drive morphological evolution as predicted by Allen's rule. However, the role of thermoregulation in the evolution of beak size across species remains largely unexplored. In particular, it remains unclear whether the need for retaining heat in the winter or dissipating heat in the summer plays the greater role in selection for beak size. Comparative studies are needed to evaluate the relative importance of these functions in beak size evolution. We addressed this question in a clade of birds exhibiting wide variation in their climatic niche: the Australasian honeyeaters and allies (Meliphagoidea). Across 158 species, we compared species’ climatic conditions extracted from their ranges to beak size measurements in a combined spatial-phylogenetic framework. We found that winter minimum temperature was positively correlated with beak size, while summer maximum temperature was not. This suggests that while diet and foraging behavior may drive evolutionary changes in beak shape, changes in beak size can also be explained by the beak's role in thermoregulation, and winter heat retention in particular.

鸟类的喙在觅食活动中发挥着关键作用，此前绝大多数针对喙大小与形态的研究均聚焦于这一功能。近期研究表明，喙在体温调节中同样具有重要意义，而这或许会如艾伦法则（Allen's rule）所预测的那样，推动形态学层面的演化。然而，体温调节在跨物种喙大小演化中的作用，目前仍未得到充分探索。尤为关键的是，学界尚未明确：冬季保温与夏季散热这两种需求，哪一种在喙大小的自然选择中占据主导地位。要评估这两种功能在喙大小演化中的相对重要性，亟需开展比较研究。我们针对一类气候生态位差异显著的鸟类支系——澳大拉西亚吸蜜鸟及其近缘类群（Meliphagoidea）解答了这一问题。我们对158个物种展开研究，结合空间-系统发育联合框架，将从各物种种群分布区提取的气候条件数据与喙大小测量值进行了对比分析。研究结果显示，冬季最低气温与喙大小呈显著正相关关系，而夏季最高气温则未表现出此类相关性。这表明，尽管取食习性与觅食行为或许会推动喙形态的演化改变，但喙大小的变化同样可以通过其体温调节功能得到解释，其中尤以冬季保温的作用更为显著。