How does global warming affects bird migration?

Broad-tailed hummingbirds may have a hard time finding food during the short breeding season as temperatures in the Colorado Rocky Mountains continue to warm steadily. bberwyn photo.

Earlier nesting and breeding observed in some species

By Summit Voice

FRISCO — Some birds are nesting and hatching earlier because of steadily increasing global temperatures, and that may be driving earlier migration in some species according to scientists with the University of East Anglia.

Changes in migration timing has already been linked with a biological disconnect between some species and their primary food sources, for example hummingbirds that fly to the southern Rocky Mountains, as well as purple martins that fly from South America to eastern North America. Both species arrival is increasingly out of synch with key food sources.

“We have known that birds are migrating earlier and earlier each year … particularly those that migrate over shorter distances,” said Lead researcher Dr. Jenny Gill from UEA’s school of Biological Sciences. “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 research team looked at a population of Icelandic black-tailed godwits over 20 years. During this time period, the flock advanced their spring arrival date by two weeks.

“The obvious answer would be that individual birds are simply migrating earlier each year. 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,” Gill said.

The team went on to investigate what could be causing the overall arrival time of godwits to creep forward.

“Because we have been following the same birds for so many years, we know the exact ages of many of them.

“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. 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.”

This can also explain why advances in migration timing are not common among species migrating over long distances. “Many long-distance migrants arrive so late on the breeding grounds that they have little opportunity to respond to warming conditions by nesting 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.”

The research team has been supported by a network of more than 2000 birdwatchers who report sightings of colour-ringed black-tailed godwits along the whole flyway, from Iceland to Spain and Portugal.

The research was funded by Natural Environmental Research Council (NERC).

‘Why is timing of bird migration advancing when individuals are not’ by Jennifer A Gill and Jose A Alves (both UEA), William J Sutherland (University of Cambridge), Graham F Appleton (British Trust for Ornithology), Peter M Potts (Farlington Ringing Group) and Tomas G Gunnarsson (University of Iceland) is published by Proceedings of the Royal Society B on November 13, 2013.


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