Every autumn, millions of birds leave the cold north and fly south. Some of them travel more than ten thousand kilometres. They do this because food becomes hard to find when winter arrives. Insects disappear, seeds are covered by snow, and small lakes freeze. In warmer places there is still enough to eat.

Before the journey, a bird eats as much as it can. Some small birds double their weight in two weeks. This extra fat is their fuel. A bird that is too thin cannot cross a desert or a sea, because there is nowhere to stop and rest on the way.

How do birds know where to go? Scientists have found several answers. Many birds follow the sun during the day and the stars at night. Others can feel the magnetic field of the Earth, like a compass inside the head. Older birds also remember rivers, mountains and coastlines from earlier journeys.

Migration is dangerous. Storms push birds off course, tall buildings and bright lights confuse them at night, and the wet places where they used to rest are often drained for farms. So people are now building small wetlands along the old routes and turning off city lights during the busiest weeks. Simple help like this saves many birds.