Iolo Williams and Chris Packham go looking for dippers in Gilfach Nature Reserve, which is rich in wildlife because no fertiliser has ever been used on the farmland. They find a pair of young dippers - a typical bird of upland streams and rivers. The young look much fatter than the adults and have fledged in the last few days. No one knows why dippers dip. One theory is that it is to attract insects, but Iolo isn't convinced.