Ep 6 of 30: Overland migrations of terrestrial mammals form some of the most impressive natural spectacles in the world. But humans have been making it more and more difficult for animals to move long distances overland. Roads and railways cause mortalities, fences block the way, growing towns and cities disrupt routes. Monty Don hears from projects in the USA designed to help the pronghorn antelope continue on its lengthy migration and how a road planned for the Serengeti might affect the wildebeest migration.