Hands up who likes the Simpsons! Some people might recognise the scene... "It's drinking the water!". Well, how does it work then?
The water evaporates from the head. This evaporation lowers the temperature of the glass head; think of when you've got a wet head on a cold windy day. The temperature drop causes some of the Methylene chloride vapor in the head to condense, a bit like steam condensing on a cold mirror surface. The lower temperature and condensation together causes the pressure to drop within the head
The pressure difference between the head and base causes the liquid to be pushed up from the base. As liquid flows into the head, the bird becomes top heavy and tips over. When the bird tips over, the bottom end of the neck tube rises above the surface of the liquid. A bubble of vapor rises up the tube through this gap, displacing liquid as it goes (the liquid has to go somewhere!).
Finally the liquid flows back to the bottom bulb, and vapor pressure equalizes between the top and bottom bulbs. The weight of the liquid in the bottom bulb restores the bird to its vertical position.
Repeat, repeat, repeat!
