Some common migrating birds (click here) – including the blackcap, (pictured above) – are staying in the UK for two weeks or more longer than half a century ago. “We knew that birds were arriving earlier in spring, but this is the first study I know of in Europe that has also tracked when they leave in autumn,” says Stuart Newson of the British Trust for Ornithology.
Newson has tapped into amateur observations of bird migrations collected over more than 50 years, starting with paper files in the 1960s and ending with 800,000 records from BirdTrack, an internet-based volunteer observation network, to study 14 common migrating birds. He found that 11 species arrive earlier and four leave later (Ibis, doi.org/bdpr).
Amateur data is best, he says. Professional ornithologists have mostly counted birds arriving at a handful of coastal observation points. And many previous studies concentrated on dating the first birds to arrive. Observations from citizen scientists avoid such biases, and cover leaving times too.
One reason for late leaving could be that females are laying more than one clutch of eggs thanks to the longer breeding season, says Tim Sparks of Coventry University, UK. Indeed, the study also shows that birds taking most advantage of earlier springs and balmy late autumns, such as the blackcap, are increasing in number.
