I can't remember the story as said by prerna, but the lines from the original book are that there is a bird that hunts for a thorn tree from the moment it leaves its nest. It does not rest until it has found it. When it finds the thorn tree it sits in the largest thorn and sings a superlative song, out sings the nightingale and the lark. The whole world stops to listen. It then dies after that song.
Prerna feels like she made a big mistake by telling anurag the thornbird story because he made her the thornbird. She was hunting for that perfect love all her life. He came into her life as that perfect allowed and encouraged her to impale herself on this love. And then destroyed her such that she will live in the pain of this destruction for the rest of her life. Always unsatisfied and heartbroken from this moment on.
Anurag thinks that he has becomes the thornbird. He has sacrificed his love for her family and for his family. And now he must live in pain for the rest of his life for that sacrifice.
Two thornbirds.