Tracing its footprints

By Sumana Narayanan
Published: Tuesday 31 March 2009

Tracking the migratory routes of songbirds is now easy

songbird populations have been declining over the last half century. Understanding their annual migration would aid conservation measures. A study has used coin-sized geolocators to track the migration of two species of songbirds. This has thrown up new information: songbirds travel much faster than previously known.

Initially information on migration was dependent on chance capture of previously tagged (with metal bands) birds. "But the probability of recapture is very low. Even after a century of tagging birds we have very limited information and on just a few species," said Oscar Gordo of the National Museum of Natural Sciences in Madrid and who has studied bird migration.

A team from US, UK and Canada has successfully mapped the migration of songbirds by using coin-sized geolocators. These geolocators were developed by the British Antarctic Survey. Smaller than a postage stamp, the device consists of a clock on a computer chip with a light detector. Strapped onto the back of the birds, these locators record levels of light. Using them, sunset and sunrise times can be figured out.

Two species of songbirds--purple martins and wood thrushes--were tagged with geolocators on their backs. "The martins are stationary at midnight and the thrushes, at noon. Based on these standards the latitude/longitude of the rest of the locations was calculated and the entire migration route was plotted," said Bridget Stutchbury, one of the authors of the paper that was published in the February 13 issue of Science.

Plotting the migratory route of the birds made recapture easy. Of the 14 thrushes and 20 martins fitted with geolocators, five thrushes and two martins were caught a year later and the geolocators were recovered.

The data showed that one of the purple martins took 43 days to reach Brazil during autumn from eastern US covering 6,921 kms. The same distance it covered in just 13 days to be back at its breeding ground during spring. Earlier it was estimated songbirds fly 150 km per day but they actually fly over 500 km a day, the study showed. The researcher also discovered that thrushes from the same population wintered near each other in Central America.

"This study shows how tools like geolocators can be used for migration studies," said Gordo. "Just conserving breeding grounds is not enough. Migratory routes need to be conserved. If there are environmental changes in their wintering areas, their population is affected," added Gordo.

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