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…
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