Sing a song of distance

Scientists have discovered that song birds fly much faster during their migrations than previously thought.

Writing in this week's Science, York University, Canada researcher Bridget Stutchbury and her colleagues tracked the progress of two species of migrating birds, wood thrushes and purple martins.  The team trapped birds in their native Pennsylvania and equipped them with tiny geolocator backpacks.  These devices continually log light levels, allowing the position of a bird wearing one to be accurately pinpointed by referring to the sunrise and sunset times, which of course vary geographically.  Once tagged the birds were released and embarked on their winter migrations to South America.

The following year, when the birds returned to their US mating sites, the team re-captured a number of them and downloaded the data from the geo-locators, enabling the routes the birds had taken, and over what time periods, to be retraced.

"Never before has anyone been able to track songbirds for their entire migratory trip," says study author Bridget Stutchbury.  Previous estimates had put the birds' speeds at at about 93 miles (150 km) per day, but the new results show that they are covering more than three times that distance.  They were also up to six times faster returning in the spring for mating that during the outward leg when there's a competitive advantage to being the first back because early arrivals have access to the best nest sites and the most food.

Even so, the researchers were surprised by the results. "We were flabbergasted by the birds' spring return times. To have a bird leave Brazil on April 12 and be home by the end of the month was just astounding.  We always assumed they left sometime in March," Stutchbury said. The researchers also found that prolonged stopovers were common during fall migration.  The purple martins, which are members of the swallow family, had a stopover of three to four weeks in the Yucatan before continuing to Brazil.  Four wood thrushes spent one to two weeks in the southeastern United States in late October, before crossing the Gulf of Mexico, and two other individuals stopped on the Yucatan Peninsula for two to four weeks before continuing migration.

This work is extremely important from a conservation perspective.  "Songbird populations have been declining around the world for 30 or 40 years, so there is a lot of concern about them," points out Stutchbury.  "Tracking birds to their wintering areas is also essential for predicting the impact of tropical habitat loss and climate change.  Until now, our hands have been tied in many ways, 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."