A warming planet is forcing some birds to fly ever further north, and this is bad news for the critically endangered Balearic shearwater, which will only nest on certain cliffs
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In the middle of 2023, a scientific survey revealed that two-thirds of bird communities in Europe had shifted northeast over the past 30 years. Scientists put this geographical shift down to climate change and birds seeking cooler climes. The survey revealed that the average distance a bird community moved was 100km (62 miles) but that inland bird communities tended to move more than coastal ones. According to the study authors this was due to coastlines acting as barriers preventing a bird community from shifting too much.
Now, a new study led by University of Oxford biologists, and recently published in the journal Proceedings of the National Academy of Sciences, has taken the theme of birds moving to new areas because of climate change, but has focused on one particular – and very rare – seabird.
The Balearic shearwater, which nests only on the coastal cliffs of Spain’s Balearic archipelago, is Europe’s rarest seabird with an estimated population of just 24,000-26,500 individuals. Classed as Critically Endangered, the population is falling by an estimated 7.4 per cent a year, which could mean that this master of the ocean winds will become extinct within 50 years.
Although the birds breed in the Balearic Islands once the nesting season is over, they migrate out into the Atlantic and head toward the fishing grounds off the coast of northern Spain as well as parts of France. But now, they’re also being increasingly seen in UK coastal waters.
Since 2010, the study team have been tracking colonies in Mallorca using miniature on-board geolocation devices. These revealed that individual birds have been migrating further and further north once they leave the Mediterranean.
It was unknown, however, whether this change was being driven by individual birds altering their behaviour, or through natural selection favouring birds that travel further.
To answer this, the researchers compared the migration tracks of the same individuals tagged in multiple years. This revealed that individual birds were shifting their range northwards by an average of 25km per year.
Co-lead author Joe Wynn said: ‘We found that the best predictor of this change in migratory behaviour was the average sea surface temperature in the summering grounds, suggesting that the birds may well be following changes in underlying marine resources. The fact that individuals can be this flexible in the face of rapid climate change is encouraging.’
But despite this flexibility in their summer destination, Balearic shearwaters are much more constrained in where they breed, so migrating further north means they have further to fly back in the autumn.
Co-author Professor Tim Guilford added: ‘We found that individuals speed up their return migration the further north they have gone, but this only partially compensates for the extra distance and they still arrive back in the Mediterranean late. We don’t yet know how such delays may affect their breeding success or survival.’
To sum up what climate change and the increasing distances Balearic shearwaters are covering in their migration might mean for the species as a whole, José Manuel Arcos from SEO/Birdlife said: ‘In addition to direct threats both on land and at sea, the increasing threat of climate change poses a challenge for a species that breeds in such a restricted habitat. Results from this study suggest that individual flexibility might help with distribution shifts driven by climate change outside the breeding season, but the question is still open about what might be the consequences of climate change for the birds during breeding, when their movements are constrained by the location of the colony.’
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