Why do bats live in caves, context clues

Some of the most SUCCESSFUL species of bats live in large cave colonies. What does successful mean in this sentence?

Some of the most successful species of bats live in large cave COLONIES. Some of these COLONIES have millions of members, even up to 20 million!

So, why don't bats fly into things at night? They use echolocation to LOCATE food and avoid obstacles.

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