Прочитайте и скажите, почему птицы и животные мигрируют из одной страны

Прочитайте и скажите, почему птицы и животные мигрируют из одной страны в иную. A lot of birds and animals move from country to country and from continent to continent during the year. Some move when summer is over and it becomes colder and some when there is no food to cat. Many birds can39;t find food under the snow, so they fly to Africa and other warmer countries. When winter comes some birds can fly more than 7000 kilometres in 3 days. They come beck in spring and never see winter.