Overview: Chris Packham, Kate Humble and Martin Hughes-Games are at the Brecon Beacons National Park to reveal how the UK's wildlife is faring this winter. The mild start followed by plummeting temperatures are setting a real challenge. The team find out how plants and animals are managing to survive, and what viewers can do to help. They also report on a surprising influx of owls, why the ptarmigan could be the UK's toughest bird, and show why winter is actually the best time of year to see some of the country's biggest wildlife spectacles. Meanwhile, Michaela Strachan reports from a swallow roost in South Africa, where millions of birds have arrived from the UK and beyond to escape the northern winter altogether.