Saving Species is back for another year of live broadcasting about the world of wildlife conservation. The first programme is a reminder that spring has sprung and the UK's most treasured migrant birds are back - the Swallows. During the winter a BBC Natural History Unit team visited Nigeria to track down a little know population of wintering swallows - and they found them. With upward of five million individuals, the sky darkened with the swirling avian biomass. We reveal how we know East Anglia is the destination of some of these West African Swallows are migrating to.