Question

The birds are flying in the sky. It is not correct why?

Answer 1

Nothing wrong with the sentence. It is about the perspective, whether we are taking the subject “birds” as a single unit or we are taking about the group of birds as a whole.
For example: The flock of birds is flying.
Here the birds are being considered as a single unit or singular or the collective noun “flock” is being used, hence we refer to it as a single unit and the subject agrees with the verb.

In the second sentence, “The birds are flying is because we are considering every bird as one individual, hence the subject is considered to be plural.

The sentence can only be wrong if “birds” is being considered as singular, which is not possible.