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• Mapping colour: scientists used nearly 100,000 citizen science observations to map and track common buzzard plumage colour across Europe.
• Unexpected patterns: buzzard colours vary geographically with patterns that defy theories linking colour to habitat and climate.
• Fading variety: over 20 years, intermediate-coloured birds increased starkly across Europe at the expense of darker and lighter ones.
• Future research: studies are needed to explore these patterns further and to assess the impact of changing diversity on the species' adaptability.
As its name suggests, the common buzzard is one of Europe's most familiar birds of prey, often spotted perched on fence posts scanning for mice and worms, or performing spectacular loop dives over fields to attract mates. In French, it goes by an equally revealing name: buse variable, or the 'variable buzzard', inspired by plumage so diverse that some individuals have been mistaken for different species. For years, nature enthusiasts across Europe have been logging buzzard sightings online – and those records have now enabled scientists at the Max Planck Institute for Biological Intelligence and their international collaborators to build the first continent-wide picture of buzzard colour – and map how it is changing.
Life in colour
Throughout the animal kingdom, colour serves many purposes, from camouflage to thermoregulation to courtship – and even within a single species, differences can be more than feather deep. Some ecological theories predict distinct patterns of colouration, such as darker birds in forests for camouflage and in colder regions for solar heat absorption. Across birds these rules broadly hold but whether they also explain colour variation in buzzards was not known.
Findings published in Ibis – The International Journal of Avian Science reveal that lighter birds tend to be found in north and central Europe, whereas darker birds are more common in Brittany and Iberia. The intermediates – neither especially dark nor light – are most abundant in south-east Europe and the British Isles, a geographic mosaic only loosely linked to the environmental factors studied.
“Strikingly, the ecological theories we tested explained very little of the colour variation in common buzzards. For instance, lighter birds tended to be more dominant in colder regions and not in warmer ones,” said Kaspar Delhey, first author and research fellow at the Max Planck Institute for Biological Intelligence. “Common buzzard colour is strongly inherited, so these patterns could instead reflect how buzzards recolonised Europe after the last Ice Age, or be related to ecological factors not yet identified, or both. As one of Europe's most widespread and colour-variable bird, the buzzard is a powerful model for understanding how such diversity is maintained and lost in wild populations, and observations made by citizen scientists have provided an incredible resource in which to explore these questions.”
Dulling diversity
The work brought together citizen observations from multiple sources, including through a dedicated portal developed by researchers Elena Kappers and Bart Kempenaers in which observers could score buzzard colour on a seven-point scale from light to dark. The team also ranked the colour of thousands more buzzard photographs contributed by the public to online nature platforms such as iNaturalist, Observation.org and Ornitho.it, assembling a dataset of nearly 100,000 observations stretching back more than two decades to the start of the millennium. The team matched sightings against satellite data on climate, vegetation and soil, and built statistical and spatial models to interpret the results.
Crucially, the approach enabled the researchers to track how colouration is changing over time. Earlier local studies had shown that intermediate-coloured buzzards tend to be fitter, surviving better and producing more offspring than birds at either colour extreme. Analysis of the Europe-wide data mirrors these findings. By 2022 intermediate-coloured buzzards made up a significantly larger share of Europe's population than in 2000, while the proportions of dark and light birds shrunk by 22 and 14 per cent respectively – showing that the variety that inspired the bird's French name has been fading.
“We found some links between environmental changes such as loss of forest cover, but these explained only part of the picture,” said Bart Kempenaers, who heads the Department of Ornithology at the Max Planck Institute for Biological Intelligence. “If this loss of colour diversity also reflects a loss of underlying genetic variation, it could compromise the species' ability to adapt to future environmental changes. Unravelling the historical, genetic and environmental factors behind the loss of colour diversity, and how colouration impacts fitness, must now be a priority – with genomics and museum specimens offering opportunities to explore both the species’ deep history and the genetic consequences of the diversity loss we are seeing today. What excites me most is what citizen science makes possible – a superb collaboration that has allowed us to explore questions that would otherwise be beyond reach.”
Dr. Kaspar Delhey
Researcher
MPI for Biological Intelligence
KDelhey@bi.mpg.de
Prof. Dr. Bart Kempenaers
Director
MPI für biologische Intelligenz
bart.kempenaers@bi.mpg.de
Environmental correlates of spatio-temporal patterns of colour variation in a bird of prey: The common buzzard (Buteo buteo)
Kaspar Delhey, Elena F. Kappers, Mihai Valcu, Christiaan Both, Bart Kempenaers
IBIS - International journal of avian science, online 7. March 2026
https://doi.org/10.1111/ibi.70041
https://www.bi.mpg.de/kempenaers - Department website
The plumage colouring of the Common Buzzard is very diverse, ranging from light to dark.
Copyright: © MPI for Biological Intelligence / Kaspar Delhey
A dataset with almost 100,000 observations from the population shows the geographical variation in d ...
Copyright: © MPI for Biological Intelligence / Kaspar Delhey
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The plumage colouring of the Common Buzzard is very diverse, ranging from light to dark.
Copyright: © MPI for Biological Intelligence / Kaspar Delhey
A dataset with almost 100,000 observations from the population shows the geographical variation in d ...
Copyright: © MPI for Biological Intelligence / Kaspar Delhey
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