From the Super Mario Wiki, the Mario encyclopedia
Waddlewing Warning!, or Acorn Plains-1, is the first course of Acorn Plains, and subsequently, the first course of New Super Luigi U. It is unlocked from the beginning of the game, and its own completion unlocks Crooked Cavern.
The level starts with a New Super Luigi U sign next to a couple of moving rocks, where the first Waddlewing appears. Next, there is a abyss with a group of moving rocks and some enemies, such as Piranha Plants. After a few Warp Pipes, four more Waddlewings appear just before the Goal Pole.
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