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.

阿伦法则（Allen's Rule）提出：恒温动物（endotherms）的附属肢体相对于体型而言，在寒冷气候中会更小，以此减少热量散失。目前针对阿伦法则的经验证据主要来自个别物种肢体大小呈现地理梯度的零星报道。种间证据仅局限于两项关于海鸟与滨鸟腿部比例的研究。本研究对214种鸟类开展系统发育比较分析（phylogenetic comparative analyses），以探究作为重要热交换位点的鸟喙是否符合阿伦法则。所涉鸟类涵盖8个多样的分类类群：巨嘴鸟、非洲拟啄木鸟、澳大利亚鹦鹉、梅花雀（estrildid finches）、加拿大鸡形目（galliforms）鸟类、企鹅、鸥类及燕鸥类。整体分析显示，所有物种的喙长与纬度及环境温度均存在极强的显著相关性，寒冷气候栖息的物种喙长显著更短。除梅花雀类群外，其余所有类群均呈现出与纬度或海拔分布相关的、符合阿伦法则的模式。四个类群（鹦鹉、鸡形目鸟类、企鹅及鸥类）中存在与温度直接相关的证据。腿部结构对阿伦法则的支持度较弱，这表明总体而言鸟类喙部可能更易受体温调节约束的影响。本研究结果为阿伦法则提供了迄今已发表文献中最强的比较支持，并证实体温调节是塑造鸟类喙部演化的重要因素。