High colour variability in animals has been associated with adaptability to environmental change. Establishing how such variability is distributed in time and space, and identifying environmental correlates, can help understand the processes driving it. Common Buzzards Buteo buteo (herein buzzards) show extensive variation in plumage colour, ranging from dark brown to largely white, yet the factors maintaining or underlying this variation remain poorly understood. Previous studies showed that intermediate‐coloured buzzards have higher fitness and have increased in abundance over time in a Dutch population. We compiled a large citizen science dataset to map buzzard colour variation across Europe, assess temporal changes and test for potential environmental drivers. Buzzard colour variation is geographically structured: light‐coloured buzzards are mainly found in North‐Central Europe, intermediates in South‐East Europe and the British Isles, and darker buzzards in Iberia and Brittany. Lighter coloured buzzards are found in areas with lower temperatures and UVB radiation, and few volcanic soils (andosols), while darker birds are more common in forested areas. Between 2000 and 2022, the proportion of dark‐ and light‐coloured buzzards declined, while intermediates increased, reducing colour variation over time. The temporal decrease in light‐coloured buzzards is stronger in regions with a strong increase in temperature. Conversely, the decrease of dark morphs and the increase in intermediates are steeper where forest cover declined. Our results suggest that photoprotection and camouflage can partly explain spatial variation in buzzard colour, and that sulphur‐rich soils may foster increased pigmentation. However, these effects only explain small amounts of colour variation ( R 2 < 0.01), suggesting that historical contingency (phylogeography) may be more important than local adaptation. Temporal increases in intermediate‐coloured buzzards at the expense of the extremes match the trend reported for a Dutch population, indicating widespread selection against colour extremes. Future studies should identify the mechanisms responsible for colour‐related variation in fitness.
Delhey et al. (Sat,) studied this question.