Travelling Birds

Each spring, thousands of birds in the Northern hemisphere migrate towards the Arctic, where they reproduce. When autumn comes, the birds migrate towards the tropics. The reverse occurs in the Southern hemisphere. For navigation, birds use astronomical reference marks such as the sun and stars. During migration, various species of birds from all the world fly very long distances -- up to several thousand kilometers. They fly over the highest mountains, oceans, deserts, and cities, facing all kinds of dangers to survive.