DreamWorks Dragons Dawn of New Riders

You unlock the ability to call in air strikes after defeating the first boss. There are a total of five dragons that you need to free for this achievement. Two of them are unlocked from the first two boss fights and the other three are unlocked by exploring the side islands.
When you are flying, hitting the back button will bring up the world map. Unexplored islands are indicated by a red parchment with a question mark and explored islands will have a green parchment with a check mark.