Aim. Avian beak morphology is a good example of how anatomical structures evolved in response to different selective pressures, such as diet and vocalizations, but also thermoregulation. The latter function was neglected until recently, but convincing evidence has been provided about the birds’ capacity to regulate heat dissipation through their highly vascularized bill. 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 7619 barn owls. The specimens were collected by 140 museums, and represent three distinct evolutionary lineages, occurring in geographically separated regions and covering the entire distribution range of the species complex: the Afro-Palearctic Tyto alba, the Australasian T. javanica, and the American T. furcata. Results. In the three lineages, 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, as small-billed individuals occurred at higher latitudes than conspecifics living closer to the Equator. Moreover, in T. furcata large-billed birds mostly occur at lower elevation closer to the Equator, bill length decreasing progressively with concomitant increase in latitude and elevation. Discussion. These findings provide evidence of repeated evolution of bill size on a global scale that is compatible with the Allen’s rule. These results suggest a role of the bill as a potential heat-exchange surface, also in nocturnal raptors, which are not directly exposed to solar radiation and whose bill shape primarily evolved to hunt and consume animal prey.