AIM: Avian beak morphology is a good example of how anatomical structures have evolved in response to different selective pressures, such as diet and vocalizations, but also thermoregulation. The last of these functions was neglected until recently, but convincing evidence has been provided regarding the capacity of birds to regulate heat dissipation through their highly vascularized bills. According to this adaptive function and coherently with the ecogeographical “Allen's rule”, which predicts smaller body appendages in colder climates, large beaks should be favoured in warm environments. Here, we tested this prediction in the cosmopolitan common barn owl group. LOCATION: World. TIME PERIOD: 1809–2017. MAJOR TAXA STUDIED: Tyto alba species complex. METHODS: We analysed the variation in bill length relative to body size according to temperature, latitude and elevation in 7,619 barn owls. The specimens were collected by 140 museums and represent three distinct evolutionary lineages that occur in geographically separated regions and cover the entire distributional range of the species complex: the Afro‐Palaearctic Tyto alba, the Australasian Tyto javanica and the American Tyto furcata. RESULTS: In the three lineages, the bill becomes larger with increasing temperature. This convergent pattern of evolution of smaller bills in colder climates is associated with a latitudinal variation in temperature, because small‐billed individuals occurred at higher latitudes than conspecifics living closer to the Equator. Moreover, in T. furcata, large‐billed birds mostly occurred at lower elevations closer to the Equator, with bill length decreasing progressively with concomitant increase in latitude and elevation. DISCUSSION: These findings provide evidence for the repeated evolution of bill size on a global scale that is compatible with Allen's rule. These results suggest a role of the bill as a potential heat‐exchange surface in nocturnal raptors, which are not directly exposed to solar radiation and whose bill shape evolved primarily to hunt and consume animal prey.

中文翻译：

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票据大小的地理变化为艾伦在世界性猛禽中的统治提供了证据

目的：鸟类喙形态是解剖结构如何响应不同选择压力（例如饮食和发声）以及体温调节而进化的一个很好的例子。直到最近，这些功能中的最后一项才被忽视，但是关于鸟类通过其高度血管化的喙调节散热的能力已经提供了令人信服的证据。根据这种适应功能并与生态地理“艾伦法则”相一致，该法则预测在寒冷气候下身体附属物较小，在温暖的环境中应该有利于大喙。在这里，我们在国际化的常见谷仓猫头鹰组中测试了这一预测。地点：世界。时间段：1809-2017。研究的主要分类群：Tyto alba 物种复合体。方法：我们根据温度、纬度和海拔高度分析了 7,619 只仓鸮的喙长度相对于体型的变化。这些标本由 140 家博物馆收集，代表了三个不同的进化谱系，它们发生在地理上不同的地区，涵盖了物种复合体的整个分布范围：非洲古北地区的 Tyto alba、澳大利亚的 Tyto javanica 和美国的 Tyto furcata。结果：在三个谱系中，喙随着温度的升高而变大。这种在较冷气候中小嘴进化的趋同模式与温度的纬度变化有关，因为小嘴个体比生活在赤道附近的同种物种发生在更高的纬度。此外，在 T. furcata 中，大嘴鸟大多出现在靠近赤道的低海拔地区，随着纬度和海拔的增加，喙长逐渐减少。讨论：这些发现为与艾伦规则兼容的全球范围内票据规模的反复演变提供了证据。这些结果表明，喙作为夜间猛禽的潜在热交换表面的作用，它们不直接暴露在太阳辐射下，其喙的形状主要是为了捕猎和食用动物而进化的。