Have you ever noticed how fast paint thinner evaporates at room temperature? It’s this little fact that makes the glass and plastic dunking birds work.
Here’s how it works: The bulb that holds the colored liquid (methylene chloride) inside has a bubble of vapor on each end. At room temperature the liquid congregates mostly toward the bottom, with the vapor evenly balanced on both ends.
However, when you wet the head, the cloth like flocking allows the water to evaporate. The evaporation cools the head, reducing the pressure of the vapor on that end, which then sucks the liquid up the bird’s neck. Eventually the bird becomes top-heavy, so the head plunges back down into the water glass.
This water is warmer than the evaporation-cooled head, which expands the vapor there, pushing the liquid back toward the bottom of the body.
But this doesn’t last long, as evaporation on the head gets going yet again, making the bird “drink” without stopping.