We know that many animals do not stay ill one place. Birds, fish and other animals move from one place to another at a certain time. They move for different reasons: most of them move to find food more easily, but others move to get away from places that are too crowed. When cold weather comes, many birds move to warmer places to find food. Some fishes give birth in warm water and move to cold water to feed. The most famous migration   is probably the migration of the fish, which is called "salmon" . This fish is born in fresh water but it travels many miles to salt water. There it spends its life. When it is cold, it returns to its birthplace in fresh water. Then it gives birth and dies there. In northern Europe, there is a kind of mouse. They leave their mountain homes when they become too crowded. They move down to the low land. Sometimes they move all the way to the seaside, and many of them are killed when they fall into the sea. Recently, scientists have studied the migration of a kind lobster  . Every year, when the season of bad weather arrives, the lobster get into a long time and start to walk across tile floor of the ocean. Nobody knows why they do this, and nobody knows where they go. So, sometimes we know why humans and animals move from one place to another, but at other times we don't. Maybe living things just like to travel. ,. The mice in northern Europe leave their mountain when   _  .
they give birth the weather is bad the place get too crowded they haven't enough food