Iolo Williams tracks wildlife in one of Wales' extraordinary landscapes - the Cambrian Mountains. In this episode Iolo heads for the Elan Valley to look for cuckoos. They're easy to hear, but not so easy to see. He also finds red squirrels that are genetically unique to the Cambrians in a remote conifer plantation. And he looks for golden plover, one of the rarest breeding birds in Wales, on the hills surrounding Plynlimon.