Why isn’t a drinking bird perpetual motion?
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Because it’s powered by the water evaporating from its head which cools the head relative to the lower body and the slight pressure differential between the top and bottom is enough so that the fluid at the bottom rises to the top and makes it top heavy enough so that it takes a dip again. In this case, the “fuel” is the water, and in order for this process to take place the head is always covered with a material like felt which can absorb and hold onto the water, otherwise it would slide off the glass tube. So every time the bird dips, it takes a bit of water from its cup, and it will hold it in the felt until it evaporates. So the water level in the cup is slowly decreasing, as in the energy being consumed is the phase shift of the water.
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