This apparent evolutionary change did not just arise from selection on individual phenotypes, but also from fitness benefits of assortative mating. Over the 20‐year study period, the proportion of individuals with an intermediate morph increased. We also found assortative mating for colour morph, whereby assortative pairs were more likely to produce offspring and had longer‐lasting pair bonds than disassortative pairs.

We found that cumulative reproductive success differed among morphs, with the intermediate morph having highest fitness. We examined differences between morphs in adult apparent survival, breeding success, annual number of fledglings produced and cumulative reproductive success. We took advantage of 20 years of life history data collected in a Dutch population to replicate earlier studies on the relationship between colour morph and fitness in this species. Previous work on a plumage colour polymorphism in the common buzzard Buteo buteo suggested heterozygote advantage as the mechanism maintaining the co‐existence of three morphs (light, intermediate and dark). How genetic polymorphisms are maintained in a population is a key question in evolutionary ecology.

Morph mod how to change variation