Walking along the muddy tracks of the River Ouse near her home a few days ago, Rebecca Stott reflects on migration.

She contemplates the lives of the Canada geese that frequently fly over her home, as well as Aristotle's own studies of bird migration - and his extraordinary life as a migrant - while considering the historic links between the migration of people and human progress.

Producer: Adele Armstrong Sound: Peter Bosher Production coordinator: Liam Morrey Editor: Richard Fenton-Smith