Persistent plumage colour polymorphism occurs in around 3.5% of bird species, with raptors showing a disproportionately high frequency of such polymorphisms. The genus Buteo has more polymorphic species than any other raptor genus (15 polymorphic species out of 25). These polymorphisms are interesting from an evolutionary perspective, because they are heritable and hence a good model for understanding mechanisms preserving genetic variation. For evolutionary models, it is important to assess whether discrete morphs exist or whether variation is more continuous. Using image analysis, we show that in Common Buzzards Buteo buteo variation is continuous and unimodal, ranging from very dark to very light individuals. Previous studies on Common Buzzards have used a classification with three discrete morphs. We compared this classification with a seven-scale morph classification used in our study. We used photographs of the same individuals taken at different ages. Even though the plumage gets somewhat darker from juvenile to adult age, morph type did not change substantially.
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Vol. 105 • No. 2