Determinants of color polymorphism in the Eurasian scops owl Otus scops

Abstract

Understanding the evolutionary forces maintaining avian color polymorphisms is a major challenge in evolutionary ecology. Aiming to give new insights into the functional basis of color polymorphism, we studied plumage color variation and its associations with fitness proxies in an individually marked population of scops owls Otus scops during 8 yr. We found a repeatable method to assign individuals to three discrete morphs, using both photography and spectrophotometry. Individuals were either grey (33%), intermediate (37%) or brown (30%). Scops owl proved to be polymorphic as the three morphs were found in the two sexes and across ages. Frequency distribution of color within the population did not vary for the two sexes during the study period, and, within individuals, color was repeatable and not explained by plumage maturation. Females of the two extreme morphs seemed to mate at random while intermediate females seemed to mate assortatively. The color of females was not related to laying date, mean fledging mass or number of fledglings per nest. Finally, intermediate females survived slightly better than females of the other morphs. Hence, pairing seems to favor intermediate males, because all females include intermediate males among their mates, and survival seems to favor intermediate females. Despite this, the proportion of intermediate individuals did not increase during the study. This fact may allude to the importance, not analysed here, of larger scale temporal and/or spatial fluctuations in selection acting on different fitness‐affecting factors, which may help to explain the maintenance of color polymorphism in the species.