Bird migration was once one of those unassailable mysteries that had baffled humankind since Aristotle. A strange hypothesis, genuinely advanced in the early modern period, was that birds flew to the Moon for winter, and barely more credible was a notion, which haunted the patron saint of British naturalists Gilbert White, that swallows buried themselves in mud.
A modern understanding really began in the 20th century, when ornithologists started to place numbered metal rings on birds’ legs. Scott Weidensaul is one of many researchers worldwide who have helped to map this avian story. He then captured the findings in his Pulitzer-nominated Living on the Wind (2003).
Yet he was also aware that these research methods had limitations. As an example, he recalls how he and others placed bands on 4,312 beautiful woodland birds called gray-cheeked thrushes. Of these, just three have ever been relocated. The yield in information is minuscule compared with the massive efforts entailed in the trapping.
The great knot, a dumpy Asiatic shorebird, makes top human athletes look like couch potatoes
In the past two decades scientists have begun to use digital geolocators that are light enough to place on the backs of small birds such as thrushes.
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