Once encountered, the Pelagornis may be soaring above water. To get it to a land-based area to safely tranquilize said Pelagornis, attack it once and chase it in the direction of land. Once you have successfully steered it to the shore, fly a distance equivalent to ten coordinate points on a map away; just to be safe. Return and wait for the Pelagornis to land. Bola it, and you have yourself a future Pelagornis.