Data from: Determinants of color polymorphism in the Eurasian scops owl Otus scops

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 years.
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 produced per nest.
Finally, intermediate females survived slightly better than females of the
other two morphs. Hence, pairing seems to favor intermediate males,
because all females include intermediate males among their mates, and
survival to intermediate females. Despite this, the proportion of
intermediate individuals did not increase during the study period. 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.