Page last updated at 10:30 GMT, Friday, 13 February 2009

Tracking reveals songbirds' route

By Mark Kinver
Science and environment reporter, BBC News

Tagged wood thrush (Image: Elizabeth Gow)
Until now, it has not been possible to tag songbirds like the wood thrush

The migration routes of two species of songbirds have been tracked for the first time, say scientists.

They fitted tags to wood thrushes and purple martins in the north-east of the US before the birds began their journey to central and South America.

Data recovered from the devices showed that the birds took much longer to complete the autumnal migration than the return journey north in the spring.

The study's findings have been published in the journal Science.

"Never before has anyone been able to track songbirds for their entire migratory trip," explained co-author Bridget Stutchbury, a biologist from York University, Canada.

She said that most songbirds were too small to be fitted with conventional satellite tracking devices, so the team mounted miniature "geolocators" on the birds.

Geolocator (Image: Bridget Stutchbury)
The geolocators measured the light levels every 60 seconds

Using data collected by the tags, the researchers calculated that the birds' spring migration was up to six times faster than the autumnal leg.

One of the tagged purple martins took 43 days to reach its overwintering site in Brazil, yet it took just 13 days to return to the breeding colony in the spring.

"We were flabbergasted by the birds' spring return times," Dr Stutchbury said. "To have a bird leave on 12 April and be home by the end of the month was just astounding.

"We always assumed they left sometime in March."

One reason why there was such a difference in journey times was because the birds made prolonged stopovers during their flights southwards.

The purple martins spent up to four weeks in Yucatan, Mexico, before continuing to Brazil, while a number of wood thrushes rested in south-eastern parts of the US before flying across the Gulf of Mexico.

The data also revealed that the birds covered a much greater distance each day than previously thought. The team calculated that the birds could fly in excess of 500km (310 miles), compared to previous estimates of just 150km (90 miles).

Light sensitive

The project tagged 14 wood thrushes (Hylocichla mustelina) and 20 purple martins (Progne subis) in northern Pennsylvania during the 2007 breeding season.

Purple martin (Image: Tim Morton)
Purple martins flew much further each day than previously thought

During the following summer, geolocators were recovered from five wood thrushes and two purple martins.

The recovered light-sensitive devices recorded each day's sunrise and sunset times, allowing the researchers to estimate a daily longitude and latitude co-ordinate for the birds.

Dr Stutchbury said the project only became feasible when she became aware of technology being developed by the British Antarctic Survey.

"They had not really been thinking of attaching [the devices] to songbirds, but when I saw the technology, I knew that we could do this."

She added that the tracking technology played a vital role in predicting what impact habitat loss and climate change would have on the region's songbirds.

"Until now, our hands have been tied because we didn't know where the birds were going. They would just disappear and then come back in the spring.

"It's wonderful to now have a window into their journey."

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