The return of the swallows has been celebrated for thousands of years as a harbinger of spring in the United Kingdom. However, it was not known until the 20th century where they went after they disappeared in the autumn. Many people – including notable scientists – believed that they hibernated on the bottom of ponds.
With the introduction of bird ringing at the beginning of the 20th century, their true destination was revealed. In December 1912, a swallow was caught on a farm in South Africa, bearing a ring that had been placed on its leg 18 months earlier in Staffordshire, UK. Because the European winters were too cold for the insects on which swallows fed, the birds headed south each winter, traveling incredible distances until the warm south of the African continent.
However, according to the British Trust for Ornithology (BTO), swallows have recently started to spend winters in the United Kingdom instead of migrating over 6,000 miles to South Africa. This major change in behavior is one of the most remarkable signs that climate change is severely dismantling a large variety of ecosystems.
“It is indeed remarkable,” said BTO’s chief executive, Professor Juliet Vickery. “We haven’t got to go back too far to remember winters when it would have been impossible for swallows to survive the freezing temperatures, but as our winters get milder it is something we may see more and more.”
BTO’s BirdTrack survey of birdwatchers has received more than 100 reports of up to 12 individual swallows that were observed during January in the warmer south and south-west of Britain, as well as parts of Ireland. Thus, with an increase of mild winters in the past few years, it appears that a small number of swallows are able to spend the winter months in Britain, instead of migrating to the south.
“To suggest that our winters would be warm enough for swallows to survive would have been unthinkable a few decades ago,” said BTO’s Director of Science, Professor James Pearce-Higgins. “But the evidence that our climate is changing is building year by year.”
According to BTO, swallows are not the only birds affected by climate change. In fact, almost a quarter of British breeding species may be negatively affected by the steady rises in temperature witnessed in the past years. Scientists are yet to understand all the implications of these fundamental changes in bird behavior caused by global warming, and to find solutions to minimize their negative impact.