Question

Why do birds fly back to their nest in the evening?

Answer 1

Majority of birds lack rod cells, which is responsible for night vision, so due to the absence of these cells they cannot see the objects clearly in less or dim light. This is the reason why birds fly back to their nest in the evening.
 

Answer 2

Majority of birds lack rod cells, which is responsible for night vision, so due to the absence of these cells they cannot see the objects clearly in less or dim light. This is the reason why birds fly back to their nest in the evening.

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