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.