Common buzzard colour diversity is declining across Europe

The common buzzard is such a familiar sight that it is easy to overlook: it might be perched on a fence post, gliding above farmland or performing dramatic looping dives during courtship.

Its French nickname translates as the “variable buzzard”, and for good reason. Its plumage ranges so widely from light to dark that different colour forms have sometimes been mistaken for separate species.

Now, years of online sightings and photographs submitted by nature enthusiasts have enabled scientists to map buzzard colour across Europe for the first time. Their analysis found that this celebrated variety is diminishing.

According to a new study, buzzards with intermediate colouring are becoming more common, while the darkest and lightest birds account for a smaller proportion of the population.

A continent-wide map of buzzard colour

The study is a significant achievement for citizen science. Researchers at the Max Planck Institute for Biological Intelligence and their collaborators assembled nearly 100,000 buzzard observations recorded between the early 2000s and 2022.

Some records came from a dedicated portal established by Elena Kappers and Bart Kempenaers, where observers assessed buzzard colour using a seven-point scale ranging from very light to very dark.

The team also rated thousands of publicly available photographs uploaded to platforms including iNaturalist, Observation.org and Ornitho.it. The researchers then matched the sightings with satellite information on climate, vegetation and soil.

This approach let them test whether familiar ecological “rules” about colour could explain the buzzards’ patterns. These include the ideas that darker birds are found in forests for camouflage, or in colder areas because they absorb more heat.

Unexpected colour patterns emerged

The resulting map showed a striking patchwork. Lighter buzzards are more prevalent in northern and central Europe.

Darker birds occur more often in Brittany and Iberia, while those with intermediate colouring are most numerous in south-east Europe and the British Isles.

The mosaic does not closely match the patterns that might be expected if the environment alone determined buzzard colour.

“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 first author Kaspar Delhey, a research fellow at the Max Planck Institute for Biological Intelligence.

If climate and habitat explain so little, what else might be responsible? Delhey identified two likely contributors.

“Common buzzard colour 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,” he said.

Drifting towards “middle” colours

The study’s most intriguing finding concerns not just where the different colours occur, but how their prevalence is changing.

Previous local research suggested that intermediate-coloured buzzards tend to fare best, surviving at higher rates and raising more young than birds at either extreme. The new Europe-wide dataset supports that finding: intermediate birds are becoming more common across the continent.

By 2022, buzzards with intermediate colouring represented a significantly greater proportion of the population than they had in 2000. Over the same period, the share of dark buzzards fell by 22%, while the share of light birds declined by 14%.

The buzzard therefore remains widespread, but the range of appearances behind its “variable” nickname is gradually contracting.

The loss of buzzard colour diversity

Bart Kempenaers, head of the Department of Ornithology at the Max Planck Institute for Biological Intelligence, said environmental change may play a role, although it cannot explain the entire pattern.

“We found some links between environmental changes such as loss of forest cover, but these explained only part of the picture,” he said.

A greater concern is what colour diversity could signify beneath the surface. If a reduction in colour types also reflects a wider loss of genetic variation, the species may be less able to adapt to future changes, whether in climate or habitat.

The team therefore wants more detailed follow-up research using genomics and museum specimens. This could show whether the colour shift is connected to declining genetic diversity, changing selection pressures or a combination of modern and ancient forces.

The citizen science angle

The study stands out because it would have been almost impossible without public participation. No single research team could have tracked buzzards throughout Europe for more than 20 years and independently built a dataset of this size.

“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,” Kempenaers said.

The buzzard’s “variable” reputation still holds: light and dark birds can both be seen. But the data suggests that Europe’s buzzards are gradually becoming more uniform in appearance. Thanks to thousands of ordinary people recording birds while out walking, scientists can now monitor that change in real time.