Barn Swallow Winter Migration Revealed: New Tracking Techniques

A combination of satellite data and chemical tests reveal a more accurate picture of where Barn Swallows migrate in the winter. A team of researchers from Environment Canada used these complementary methods to track individual birds and large groups.

Like many birds, Barn Swallows (Hirundo rustica) travel south for the winter, generally to areas across Central America, the Caribbean, and South America. When they are breeding, however, they spend their time in North America where they build cup-shaped mud nests hidden within human-made buildings, such as barns.

In the latest study, Keith Hobson and Kevin Kardynal of Environment Canada investigated where exactly Barn Swallows were escaping to in the winter. Their research involved a isotope analysis of chemical elements present on the birds' feathers, which ultimately provides clues about where the birds were when those feathers grew. The birds were also fitted with small geolocators, so that researchers could track their travel, according to a news release. However, the birds must return in order for researchers to collect information from the geolocators, so combining these methods improves their overall accuracy. 

"I think the study nicely demonstrates the need to get away from the dichotomous thinking of pitting one technique against another. All are useful and all have their strengths and weaknesses if you take the time to understand them," Hobson said in the news release. "On a more serious note, we also need to remember that describing connectivity patterns is a very preliminary step in using such information to help conserve species and populations. I think there is a tendency these days to move on after we have produced some pretty maps."

In total, researchers collected feathers from 208 Barn Swallows and were able to recapture 14 swallows fitted with geolocators. When evaluating hydrogen, carbon and sulfur isotopes, researchers got a clearer picture of whether a bird spent the winter in grasslands or forests, along the coast or more inland. That's because varying hydrogen isotope ratios, for example, shed light on the body of water these birds nested near. When combining the isotope analysis with the data collected from returning swallows' geolocators, researchers were able to narrow down where these birds wintered. Overall researchers discovered most of the birds preferred open agricultural and savanna regions outside of the Amazon basin.

If birds breed and migrate together to a certain area, rather than dispersing across South America - a pattern known as migratory connectivity - then that population could be wiped out fairly quickly.

Their study, recently published in The Auk: Ornithological Advances, could help protect the birds' wintering grounds.

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