Migrating animals have for a long time been tracked by recapturing tagged individuals. More recently, radio-telemetry, which requires an observer to physically follow the tagged animal, has been used.

Animal migration : a context for using new techniques and approaches / Keith A. Hobson and D. Ryan Norris -- An introduction to light stable isotopes for use in terrestrial animal migration studies / ...

The annual spectacle – a magnet for eco tourists – has seen a “dramatic” decline over the last five years, warn ...