Autumn is the season when the landscape is at its most colourful. Iolo Williams finds himself in the midst of a flock of thousands of fieldfares arriving from Europe to escape the colder continent. They gorge themselves on berries in trees surrounding the smallest church in Wales. In the Usk Valley, bats feed before they hibernate in caves, and migrating ducks gather on Talybont Reservoir, ready for winter. Underground, cave spiders are lurking, and sea trout are heading upstream to spawn in the rivers.