In what way does he over romanticize birds and nature?

The speaker romanticizes nature by seeing it as a joyful and even peaceful place where all is right with the world,

The budding twigs spread out their fan,

To catch the breezy air;

And I must think, do all I can,

That there was pleasure there.

If this belief from heaven be sent,

If such be Nature's holy plan,

Have I not reason to lament

What man has made of man?

In reality, animals are killing each other all the time but the speaker rejects this reality for a more comforting view of the simple beauty nature provides him.