In the books, the bird is a crow not a raven. The three-eyed crow. But the series changed it to a raven.
Right from the first book that came out in 1996, Bran started having dreams about this three-eyed crow and his messeges to Bran. So then Bran, Hodor and Jojen Reed set out north to find him. They did find him and Bran became a greenseer (a person who can perceive future, past or distant events). Bran is also a warg who can enter the mind of animals and, as it turned out, also some humans like Hodor. All this was known to the writers of GoT as well as all book readers. No new book as been released during the series 8 year run. GRRM is really dragging his feet. He has told the GoT writers his plans for the end years ago. The actors found out about 2 years ago.