Brett Smith for redOrbit.com – Your Universe Online
Scientists have been seeing some bird species migrate earlier and earlier each year and now a team of UK and Icelandic researchers has shown that warming temperatures are behind the creeping back of this instinctive behavior, according to their report in Proceedings of the Royal Society B.
“We have known that birds are migrating earlier and earlier each year – particularly those that migrate over shorter distances,” said report author Jenny Gill, a biologist from the University of East Anglia in the UK. “But the reason why has puzzled bird experts for years. It’s a particularly important question because the species which are not migrating earlier are declining in numbers.”
The study focused on a population of Icelandic black-tailed godwits that advanced their spring arrival date by two weeks over the span of 20 years. Black-tailed godwits are large shorebirds that have a sporadic breeding range that extends from Iceland to Russia. The birds feed on grasslands in spring and some populations migrate to muddy waterways in Africa and India after breeding. Flocks from Iceland spend the winter in the UK, France and the Netherlands, with some birds heading to Spain or Portugal.
“The obvious answer would be that individual birds are simply migrating earlier each year,” Gill said. “But our tracking of individual birds shows that this is not the case. In fact individual birds do almost exactly the same thing every year – arriving punctually at the same time year-on-year.”
She added that because the team followed the same flock for so many years, they knew the precise ages of many individual birds.
“We found that birds hatched in the late 1990s arrived in May, but those hatched in more recent years are tending to arrive in April,” Gill said. “So the arrival dates are advancing because the new youngsters are migrating earlier.”
“Climate change is likely to be driving this change because godwits nest earlier in warmer years, and birds that hatch earlier will have more time to gain the body condition needed for migration and to find good places to spend the winter, which can help them to return early to Iceland when they come back to breed,” she added.
The study researchers said this would also explain why the same change in migration isn’t seen in birds that migrate over long distances, as long-distance migrants arrive so late that they have minimal opportunity to acclimate to warming conditions and nest earlier.
“This research is very important because many long-distance migrant bird populations are currently declining very rapidly, and identifying how climate change is affecting these populations is a key part of understanding the causes of these declines,” Gill said.
The International Union for the Conservation of Nature has black-tailed godwits listed as ‘Near Threatened’ on its Red List of endangered species.
“Although this species is widespread and has a large global population, its numbers have declined rapidly in parts of its range owing to changes in agricultural practices,” a statement on the organization’s website said. “Overall, the global population is estimated to be declining at such a rate that the species qualifies as Near Threatened.”