How do scientists follow a bird across the planet?

To follow a bird around the world, scientists have used a ladder of ever-smarter tags. The oldest is the ring. Bird ringing, or banding, attaches a small numbered metal or plastic tag to a wild bird's leg or wing so that it can be identified later. When someone finds the bird and reports the number, that is called a ringing recovery.

Scientific ringing began in 1899, when a Danish schoolteacher, Hans Christian Cornelius Mortensen, put aluminium rings on starlings. He had tried zinc earlier but found it too heavy. Rings have to be light, because the method only works if ringed birds behave like unringed ones.

You may have heard that Audubon started it all by tying silver threads on young phoebes in 1805 and seeing two come back. That story is of questionable authenticity. The return rate he reported, 40 percent of nestlings, is an extreme outlier compared with modern data of about 1.3 to 1.6 percent.

A ring has a limit: it needs the bird to be found again, so the data are limited to where the bird was released and where it turned up. A ringed Arctic tern from the Farne Islands reached Melbourne in three months from fledging, a journey of over 22,000 km, but ringing told us only the start and the finish.

Newer tags fill in the middle. A geolocator records light levels and works out location from day length and the time of solar noon. One such tag showed another tern flying 96,000 km in ten months. Satellite tags go further: scientists neither have to follow the animal nor recover the tag to get the data.