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