How does it work?
When untouched, the vapour and the liquid inside the tube are in an equilibrium that allows the bird to rest in an upright position. To change that, we dip the bird's head that is covered in fabric in water. As the water evaporates from around the head, it takes energy with it, and the head cools down.
The vapour inside the head cools and contracts. Since the glass around the vapour won't contract, a vacuum is created inside the head of the bird. The liquid is the only thing that can give, and it does. Liquid from the lower half of the bird is sucked up into the head of the bird.
The head of the bird is now too heavy for it to stay upright, and the bird dips forward. As it dips, a corridor is opened up between the head and the body of the bird. The vacuum can more easily suck the vapour than the liquid and vapour travels to the head until the pressure is equalized. The liquid drains back to the body of the bird, making the lower bulb heavier. The bird is tipped backwards and returns to its full upright position.
That is, until more water evaporates from fuzzy head and the cycle is started over. By allowing the beak of the bird to dip in water, there is a continuous supply of water soaking into the head of the bird