To warp to the Birdman stage, fly under the bridge on the first level. Grab the star and you'll be free as a bird. You can also become the birdman by flying to New York (on Little States Island) with the rocketbelt and touching the golden star in Central Park. In Everfrost Island use the rocket belt in the first test, look to your side and you will see waterfalls. Go to the right most waterfall and go to the cavern. Keep going until you see another waterfall, fly down and you will see the star that warps you to the birdman stage.