Dressed for the weather: Tawny owl feather adaptations across a climatic gradient

Populations are presumed to be adapted to local environmental conditions via natural selection, with gene flow breaking up local adaptations. In birds, several aspects of feathers may reflect local adaptation; the insulation capacity of feathers may be greater in cold regions, and colouration, as darker feathers absorb more heat compared to lighter-coloured feathers. We studied feather properties (plumulaceous part of the feather, density of barbs and barbules) of tawny owls across nine populations covering a large part of the species’ European range (9-52 individuals per population) as well as their plumage colour, scored as dark (brown) vs light (grey) morphs. We compared these traits' phenotypic divergence (PST) with the divergence expected based on genetic drift (FST) inferred using eight microsatellites. The FST was low (0.022; 95% CI 0.005-0.039), and most feather structures' phenotypic divergence (PST) exceeded the FST. However, phenotypic divergence in plumage colour was low and not significant, implying a limited role of natural selection in shaping variation in plumage colouration at large spatial scales. Between-population differentiation in feather properties was more pronounced in chest feathers than back feathers. In colder populations, the plumulaceous part of the back, but not the chest, was larger (implying greater insulation). Although proper evaluation hinges on understanding how insulative properties confer a fitness advantage in a given environment, our findings imply that properties of avian feathers may reflect local adaptation, possibly related to climate.