题目内容：

[主观题]阅读下面短文,根据短文内容,在短文后81£­85小题中句子空白处填入适当的词语或句子。We know that m

阅读下面短文，根据短文内容，在短文后８１－８５小题中句子空白处填入适当的词语或句子。

We know that many animals do not stay in one place. Birds, fish and other animals move from one place to another place 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 crowded.

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 fish, which is called “salmon”（鲑鱼）. This fish is born in fresh water but it travels many miles to salt water. There it spends its life. When it is old, it returns to its birthplace in fresh water. Then it gives birth and dies. 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 of lobster(龙虾). Every year, when the season of the bad weather arrives, the lobsters get into a long line and start to walk across the floor of the ocean. Nobody knows why they do this, and nobody knows where the go. So, sometimes we know why humans and animals move from one place to another place, but at other times we don’t. Maybe living things just like to travel.

81. The reason for most of the animals to move is _____________________.