Hey I have a question about the sentence:

A flock of birds is flying in the sky.

I understand that flock is collective and acts as a singular subject.

This is not my problem, It's the phrase flock of birds, Is there a phrase "flock of bird".

and if there is then what sentences can we apply this phrase? but if there isn't, why? is it wrong to say "flock of bird" without the "s"?

Think about it: can you have 'a crowd of person'?