Light-detecting backpacks record the complete migration routes of songbirds

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In the summer of 2007, thirty-four travellers left home with backpacks in tow to see the world. But these weren’t human students, out to get drunk and pretentious find themselves in foreign lands – they were small songbirds, migrating to tropical climates for the winter.

Their backpacks were light-measuring devices called “geolocators”, each about the size of a small coin. By measuring rising and falling light levels, these miniature contraptions revealed the timings of sunrise and sunset wherever the birds happened to be flying. Those, in turn, revealed where they were in the world, and allowed Bridget Stutchbury from York University, Toronto to achieve a world-first – track the entire voyage of a migrating songbird, from the start of the outbound trip to the end of the return journey.

The recordings show that tiny wood thrushes and purple martins are far more capable fliers than anyone had thought. They can cover 500 kilometres in a day, flying more than three times as fast as previously expected. Previous studies had credited these tiny fliers with top migration speeds of just 150 km/day. But these had major flaws.

Songbirds are so small that they can’t be tracked by satellites, making their annual migrations difficult to track. Until now, what we knew about their journeys came from brief glimpses on radar or studies done at pit-stops along the way. One incredible study managed to track thrushes during a short part of their travels by injecting them with mildly radioactive isotopes and following them in a plane. All of these studies have provided mere glimpses of the overall migration, like piecing together a movie from still shots and trailer clips. Stutchbury’s team from the University of York, Toronto have managed to record the entire film.

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They captured 14 wood thrushes and 20 purple martins in northern Pennsylvania and fitted them with geolocators. One year later, in the summer of 2008, they managed to recapture five of the thrushes and two of the martins, and analysed the data stored in their mechanical backpacks. These recordings revealed an aeronautical skill that has been greatly underestimated.

Both purple martins flew from the eastern United States across the Gulf of Mexico to the Yucatan Peninsula. They covered the 2,500 km distance in just five days, travelling at a speed of just 500 km/day. There, they stopped for about 3-4 weeks before carrying on to Brazil. The thrushes took a similar route but had a short stopover in the southeastern United States before moving on to a very narrow region of Nicaragua and Honduras, where they spent the winter.

Both species flew faster on the return trip than the outbound one. One particularly speedy martin left the Amazon on the 12th of April and was home by the end of the month. It covered the 7,500 km trip home in just 13 days, nine of which in spent on the wing. That gave it an astonishing flight speed of just under 600 km per day. The wood thrushes also flew home in 13-15 days, but with less ground to cover, they did so more leisurely than the martins.

Charting the journeys of small birds is much more than a birdwatcher’s fancy – it will be an increasingly important part of conserving these charismatic songsters. The populations of many songbird species are crashing in North American and Europe and their dependence on distant parts of the world for shelter and food makes them vulnerable to global shifts in climate.

These threats make it ever more urgent to work out exactly where they go when they migrate and the routes they take to get there. For example, we now know that wood thrushes, one of the species in decline, spend winter in a very narrow band of forest in Nicaragua and Honduras. This area will be a clear staging ground for any future efforts to conserve this species. Only with information like this can we hope to understand how habitat loss and climate change will affect them in the future and only then can we make sensible plans to ensure their survival.  

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Reference: Science  10.1126/science.1166664

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