Correct sentence (simple and clear):
When it gets cold, many birds fly away to find food.
Why this is better (easy steps):
- When it gets cold tells when something happens. We put a comma after it.
- many birds is who does the action.
- fly away is the action. We use "fly" (simple present) because this happens regularly when it gets cold, not only right now.
- to find food explains why they fly away. It’s a short and clear way to say their purpose.
Note: Your original sentence was okay and people would understand it. Another good version is: When it gets cold, many birds fly away to look for food.
Good job noticing the sentence! Keep practicing little changes like this to make sentences clearer.