Geographical variation in bill size provides evidence for Allen’s rule in a cosmopolitan raptor

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.