image credit Dawn Perry). He noticed that the energy wasn’t going anywhere and the inventor determined that something could be done with them. What he wanted to do was to harness motion- the idea of the bird came along later to make it more entertaining. Of course, while it is simply for fun it is the science behind it which makes it work.
The contraption is what is known as a heat engine. This is something that can be built when there are temperature differences that create energy. To start the bobbing the head of the bird must be unceremoniously dunked in water. The bottom bulb is room temperature. The heat there raises the vapor pressure of the liquid inside to a higher degree than it is up in the head.
Many assume that the liquid is colored water but it is Dichloromethane, an industrial paint stripper, degreaser and solvent. This organic compound is used since it has an accelerated rate of evaporation. Methylene chloride will irritate the skin and lungs but as long as the glass does not break, you are safe. If it does spill, ventilate the room well and it will soon evaporate.