This is called the Crow's Nest... where Yarr keeps a lookout. You can get a great view... of the island!
The Crow's Nest was the highest point on The Migrator, and served as a lookout point for Rockhopper and Yarr, as well as a host for the Snow Cannon 3000, which was the main means of defense for the ship. Yarr could often be seen here when the Migrator was docked.
The Crow's Nest was quite sparse, and contained simply the Snow Cannon 3000, some ammunition, and a large jolly roger flag in the center. There was a railing surrounded the entirety of the Crow's Nest, and there were shrouds which served as a ladder of sorts, between the nest and deck.
During Holiday Parties from 2008 to 2012, the railing around the Crow's Nest had green trim and Christmas lights. The pile of snowball ammunition was also replaced by a large snowman that looked like Rockhopper.
Before the Adventure Party, plants were growing up to the Crow's Nest, from the deck.