Data from: Geographical variation in bill size across bird species provides evidence for Allen’s rule

Allen’s rule proposes that the appendages of endotherms are smaller, relative to body size, in colder climates, in order to reduce heat loss. Empirical support for Allen’s rule is mainly derived from occasional reports of geographical clines in extremity size of individual species. Interspecific evidence is restricted to two studies of leg proportions in seabirds and shorebirds. We used phylogenetic comparative analyses of 214 bird species to examine whether bird bills, significant sites of heat exchange, conform to Allen’s rule. The species comprised eight diverse taxonomic groups - toucans, African barbets, Australian parrots and estrildid finches, Canadian galliforms, penguins, gulls and terns. Across all species there were strongly significant relationships between bill length and both latitude and environmental temperature, with species in colder climates having significantly shorter bills. Patterns supporting Allen’s rule in relation to latitudinal or altitudinal distribution held within all groups except the finches. Evidence for a direct association with temperature was found within four groups (parrots, galliforms, penguins and gulls). Support for Allen’s rule in leg elements was weaker, suggesting that bird bills may be more susceptible to thermoregulatory constraints generally. Our results provide the strongest comparative support yet published for Allen’s rule, and demonstrate that thermoregulation has been an important factor in shaping the evolution of bird bills.艾伦法则（Allen’s rule）提出，内温动物（endotherms）的附肢相较于躯体尺寸，在寒冷气候中会更小，以减少热量散失。有关艾伦法则的实证支持，主要源于个别物种肢体尺寸呈现地理梯度的零散报道。种间层面的支持证据仅局限于两项针对海鸟与滨鸟腿部比例的研究。本研究通过对214种鸟类开展系统发育比较分析（phylogenetic comparative analyses），检验作为核心热量交换位点的鸟类喙部是否契合艾伦法则。所涉类群涵盖八大多样化的分类单元：巨嘴鸟、非洲拟啄木鸟、澳大利亚鹦鹉、梅花雀科雀类（estrildid finches）、加拿大鸡形目鸟类（galliforms）、企鹅、鸥类与燕鸥。整体分析结果显示，所有物种的喙长均与纬度及环境温度存在极强的显著相关性，寒冷气候分布的物种喙长显著更短。除梅花雀科雀类外，其余所有类群均呈现出与纬度或海拔分布相关的、契合艾伦法则的分布模式。在鹦鹉、鸡形目鸟类、企鹅与鸥类这四个类群中，检测到喙长与温度存在直接关联的证据。针对腿部结构的艾伦法则支持证据则相对薄弱，这提示总体而言鸟类喙部或许更易受到体温调节约束的影响。本研究结果提供了迄今已发表文献中最为强有力的艾伦法则比较研究支持，并证实体温调节一直是塑造鸟类喙部演化的关键因素。