Scops owls' datasets to study the fitness consequences of assortment based on coloration and body size

The persistence of color polymorphism in nature, essential for maintaining genetic diversity, may be driven by disassortative mating based on color. In birds, body size often correlates with coloration, complicating mating patterns, as selective pressures favoring mixed-color pairs might counterbalance sized-based selection. This complexity is heightened in species with reversed sexual size dimorphism, such as owls, where smaller, agile males are likely favored for foraging efficiency. Here we investigate the fitness consequences of assortative mating by color and body size using a long-term dataset from the color polymorphic Eurasian Scops owl (Otus scops), a species with reversed sexual size dimorphism. Results reveal that size-assortative mating enhances reproductive success, as pairs with greater size similarity—particularly larger pairs—produce fitter offspring, a finding that challenges expectations under reversed sexual size dimorphism. Although body size and plumage coloration were correlated within pairs, only size similarity, not coloration, contributed to higher fitness. While color-based assortative mating had no direct impact on fitness, greyer pairs (usually smaller) exhibited higher feeding rates to offspring than browner pairs. These results underscore the importance of considering the adaptive value of body size and its covariation with color to understand the persistence of color polymorphisms in nature.