Geographic variation in body size and plumage colour according to diet composition in a nocturnal raptor

Predator–prey interactions are amongst the strongest selective forces that promote the evolution of local phenotypes in both predators and prey. However, intraspecific spatial covariation in phenotypic traits between predators and prey has been rarely investigated, especially at a large geographic scale. Here, we studied the covariation between prey composition and some phenotypic traits, such as wing length, bill length and plumage colour, of a widely-distributed nocturnal predator, the western barn owl Tyto alba. By using 3100 specimens collected across its entire range of distribution, spanning from Europe to Middle East and Africa, we showed that wing length positively covaries with prey size, but not with taxonomic composition. This finding suggests that larger prey might have selected for larger body size and/or that larger individuals might be more selective in hunting large prey. In addition, we also found that paler-plumaged populations generally hunt larger prey. Paler barn owls might be thus better specialized in capturing averagely larger prey and/or mainly hunt in habitats where larger prey are more abundant. In addition, considering that paler individuals are generally larger than brownish ones, it is possible that paler plumage colour might have evolved as a by-product of selection towards a large body size, which in turn have emerged in response to prey size composition. However, irrespectively of the direction of causality and the phenotypic target of selection, we showed that predator–prey interactions can affect spatial phenotypic variation by promoting the evolution of local adaptations.