One of the longest-running animal research projects in the world is 50 today. The Bewick's swan study was started at the Wildfowl and Wetlands Trust at Slimbridge in Gloucestershire by Sir Peter Scott in 1964.
He noticed that the birds have unique bill patterns, which researchers have to learn before they can take part.
More than 9,000 different swans have been identified. The work has provided data on survival rates and meant breeding sites in Russia have been protected.