This is the famous dunking duck. Place a glass of water so the beak can be tipped in and the duck will bob up and down on its own. Water on the beak evaporates. This cools the beak lowering the pressure. The fluid is pulled up into the beak. The center of mass shifts causing the bird to tip into the glass of water. The pressure equalizes allowing fluid to return to the lower bulb. The bird returns to upright. The process repeats. Mechanics, fluids and thermo-dynamics are needed to understand this phenomenon.

Dunking Duck