Mark Carwardine joins thousands of other bird watchers taking part in the first ever "Migration Watch", run by the British Trust for Ornithology.
They are mapping out the movements of migratory birds including swallows, swifts and cuckoos as they move up through the country. He also explores the latest research, using unique wind-tunnel experiments at Lund University in Sweden, into how these small migrant birds are able to fly such huge distances. Repeat of yesterday 9pm