Data from: 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
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.