Every year birds of all varieties migrate from country to country, or even from continent to continent, in their bid to follow the good weather and find a mate. But how can scientists track the progress of endangered species to ensure that they can migrate safely for years to come? Quentin Cooper talks to experts who follow birds by using simple tags, radio transmitters and even by measuring the radioactivity in birds' toenails.
Producer Sarah Empey EMAIL: material.world@bbc.co.uk