Beak was a mutant whose bones were light and hollow. His arms had begun to grow feathers, which when fully formed, may have allowed him to fly. He could at the mean time glide and fly for short distances with great effort. Beak also had enhanced eyesight along with talons on his hands and feet. Even though Beak had bird-like qualities, it was extremely difficult for him to fly.
You can read more about Beak here!
He’ll be joining Hugh Jackman (Wolverine), Liev Schreiber (Sabretooth), Ryan Reynolds (Deadpool), Taylor Kitsch (Gambit), Will.i.am (Kestrel), Danny Huston (William Stryker) and Lynn Collins (Silver Fox) in the Gavin Hood-directed pic, opening May 1, 2009.
Source: Just Jared