Evaporation of water on duck's head cools vapor inside causing low pressure, etc.
How it works:
Dippy Duck is a small heat engine consisting of a hollow glass barbell with opposite ends able to seesaw about a knife edge pivot. One end of the barbell is filled with a high vapor pressure liquid. The other end is empty on the inside and coated with absorbent flocking on the outside.
When the flocking is wet, evaporative cooling reduces the air pressure inside the empty end of the barbell, causing the liquid at the other end to get sucked up into it. As the liquid rises it changes the center of mass of the barbell, causing the top end to slowly dip down. Eventually enough liquid enters the top of the barbell so that it dips to a nearly horizontal position, allowing the air pressures at the ends of the barbell to equalize. Gravity pulls all the liquid back down into its original reservoir, and the Dippy Duck returns to its nearly vertical orientation. This process repeats as long as water is able to evaporate off the flocking and the liquid inside remains warmer than the air outside.
Once there was a dippy duck over a foot tall, gone now and no apparent replacements on the market. A dozen ducks each six inches tall goes a certain distance.