Question

the nictiating membrane present in frogs and birds serves as covering membrane for eyes but this is vestigial in human.why?

Answer 1

The answer to your question is pretty simple. Nictitating membrane is like an extra eyelid and what does an eyelid do?? It protects your eye!!!!
Similarly the nictitating membrane protects the eyes of frogs and birds from extreme weather, excess water etc., whereas humans don't have to suffer through this anymore.
Therefore, nictitating membrane is a vestigial organ in humans.