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.