Tucked in behind Dorset's famous Chesil Beach is a unique and beautiful place - the Fleet Lagoon. Martha Kearney explores a 1,000-year history of human guardianship of birds on the lagoon. On the way she gets a close-up view of little tern dating platforms on the Fleet itself, learns why the swans of the ancient Abbotsbury Swannery are much safer now and who they hate the most, and learns to build a nest