Europe's buzzards are losing their color diversity

The plumage colouring of the Common Buzzard is very diverse, ranging from light to dark.
Photo Credit: © MPI for Biological Intelligence/ Kaspar Delhey

Scientific Frontline: Extended "At a Glance" Summary: Loss of Colour Diversity in Europe's Common Buzzards

The Core Concept: The common buzzard (Buteo buteo), historically recognized for its highly variable plumage, is undergoing a continent-wide homogenization in color. Intermediate-colored birds are increasingly dominating the European population at the expense of both lighter and darker variants.

Key Distinction/Mechanism: While standard ecological theories predict that plumage color correlates strongly with specific environmental factors—such as darker feathers for forest camouflage or for heat absorption in colder climates—buzzard coloration largely defies these rules. Instead, the color shift is driven by the inherently higher survival and reproductive fitness of intermediate-colored individuals, operating across a geographic mosaic that likely reflects post-Ice Age recolonization patterns rather than immediate environmental demands.

Origin/History: This demographic shift was identified using a dataset of nearly 100,000 citizen science observations stretching back to the year 2000. Researchers established that by 2022, the proportions of dark and light buzzards in Europe had shrunk by 22% and 14%, respectively.

Major Frameworks/Components: 

• Citizen Science Integration: Aggregation and standardized scoring of observational data and field photographs from public nature platforms (e.g., iNaturalist, Observation.org).
• Spatio-Temporal Modeling: Cross-referencing extensive bird sightings with satellite data on regional climate, vegetation, and soil to build accurate spatial distribution models.
• Ecological Theory Testing: Evaluating—and subsequently challenging—traditional hypotheses regarding thermoregulation and camouflage as the primary drivers of localized coloration.
• Genomic and Historical Analysis: The planned integration of genomic testing and museum specimens to evaluate the historical genetic consequences of modern diversity loss.

Branch of Science: Biology, Ornithology, Ecology, and Evolutionary Genetics.

Future Application: The analytical frameworks utilized in this study provide a scalable model for leveraging massive crowdsourced datasets to track phenotypic, ecological, and evolutionary shifts in other wild species over time.

Why It Matters: The visible loss of plumage diversity likely reflects a deeper loss of underlying genetic variation. A reduction in genetic diversity directly threatens the evolutionary resilience of wild populations, potentially compromising the common buzzard's ability to adapt to impending climate and environmental changes.

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 color – and map how it is changing. 

A dataset with almost 100,000 observations from the population shows the geographical variation in dark, light or intermediate plumage colouration of the Common Buzzard in Europe.
Image Credit: © MPI for Biological Intelligence/ Kaspar Delhey

Life in color 

Throughout the animal kingdom, color 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 coloration, 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 color variation in buzzards was not known. 

Findings 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 color 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 color is strongly inherited, so these patterns could instead reflect how buzzards recolonized 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 color-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 color on a seven-point scale from light to dark. The team also ranked the color 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 coloration is changing over time. Earlier local studies had shown that intermediate-colored buzzards tend to be fitter, surviving better and producing more offspring than birds at either color extreme. Analysis of the Europe-wide data mirrors these findings. By 2022 intermediate-colored 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 color 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 color diversity, and how coloration 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.” 

Published in journal: International journal of avian science

Title: Environmental correlates of spatio-temporal patterns of colour variation in a bird of prey: The common buzzard (Buteo buteo)

Authors: Kaspar Delhey, Elena F. Kappers, Mihai Valcu, Christiaan Both, and Bart Kempenaers

Source/Credit: Max-Planck-Gesellschaft

Reference Number: bio031026_01

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