Tawny Owl females of intermediate colour morph reproduce better and increase in a local polymorphic population

Abstract

Individuals of the same population with differing melanin plumage colouration can respond differently through fitness to environmental stressors. In a local population of Tawny Owls (Strix aluco), we collected individual-based data on nesting attempts of this well-known colour polymorphic species between 1985 and 2014. We studied whether body size, condition, demographic parameters and habitat around occupied nest sites differ among grey, intermediate and brown coloured females. The proportions of females belonging to each of the three colour morphs in the local population changed during the study period, withintermediate individuals becoming more frequent in the last decade. Females were similar in body size and achieved similar body condition during breeding irrespective of plumage colouration. Although most of the analysed demographic parameters were not statistically significantly related to the colour morph, the intermediate owls raised more nestlings in a lifetime and tended to have higher recapture probability. We found little support for habitat and morph associations. In summary, we propose that the increase in intermediate Tawny Owl colour morphs in this local population is associated with their better reproductive performance compared to the other morphs.