As we enter Holy Week, Mark Tully explores birds as symbols of spiritual hope in this week's Something Understood. Soaring above the earth, for many poets and composers they have come to represent the soul, freed from the constraints of our earthly form. Mark travels to Suffolk to Lakenheath Nature Reserve, and with hobbies soaring in the background, meets nature writer Richard Mabey, widely respected as one of the leading experts on British birds. He's also the author of "Nature Cure", a book about his recovery from a deep depression, and he talks movingly about his mixed emotions at this time of year, as Spring arrives. With poetry by Thomas Hardy, George Herbert and Isaac Rosenberg, and music from Handel to Miles Davis - all celebrating the unexpected joy birds can bring. The producer is Elizabeth Burke, and this is a Loftus production for BBC Radio 4.