From:
http://www.backstreet.demon.co.uk/oddstuff/drinkingbirds/drinkingbirds.htm
ANATOMY OF A HAPPY DRINKING BIRD -
HOW IT WORKS - The techie stuff.
The body of the bird comprises 2 glass bulbs, one for the head and one for the lower body.
The glass tube which interconnects the two bulbs dips deep into the liquid (usually coloured methylene chloride) in the body.
The head is coated in a felt-like material which absorbs water when the bird "dips". Evaporation of water from the head plays a key part to the operation of the bird, because it causes the head to become cooler than the body. The swaying motion of the bird assists the evaporation.
Although the head and upper part of the glass tube appear to be "empty", they are actually full of vapour from the methylene chloride. Methylene chloride is good for this because it doesn't take much heat energy to turn it into a vapour.
Because the head is cooler than the body, some of this vapour condenses inside the head, like steam when it touches a cold window. As this vapour "shrinks" into minute droplets of fluid, it takes up a lot less space. This makes the pressure inside the head slightly lower than the body, causing the liquid to be sucked up the tube. You could think of it as the "hot" fluid in the body making "steam" above itself, which blows the liquid up the tube (vapour pressure); it's all relative. The main thing is, the body is always warmer than the head. It's not the same a thermometer, though, because it does not rely on expansion of the liquid itself, which is insignificant. It's the pressure of the vapour that does the work.
As the liquid rises up the tube, it gradually changes the centre of gravity of the bird. This makes it tip over more and more until eventually it tilts into the water. If everything is adjusted just right, then as it tilts over, the end of the tube inside the body comes out of contact with the liquid. Instead of pushing the liquid up the tube, the vapour above the liquid in the body can now quickly rush up the tube, equalising the pressures in head and body. As this happens, the liquid which has moved up towards the head now gurgles back down into the body. This rapidly moves the centre of gravity back to the lower body, and the bird swings back away from the glass.
