Lake of Wisdom

The Lake of Wisdom is a lake in front of the Mighty Eagle's cave, at Bird Island. When he stayed at the cave there, Mighty Eagle commonly urinated in it.

The Angry Birds Movie
After discovering the Mighty Eagle's cave, Red, Chuck, and Bomb the saw the legendary lake. Chuck and Bomb then gargled, swam, and drank the water of the lake, much to Red's displeasure. After hearing the footsteps of Mighty Eagle, they looked up in awe at the legendary hero, before discovering he actually urinates in the lake, much to their disgust.

Trivia

 * The lake appears along with the cave in The Eagle's Den in Pig Days (Angry Birds Seasons).