The bird pivots on glass tubes. Water on the absorbent beak evaporates, cooling the beak. The red liquid in the body is drawn up because of the partial vacuum caused by the cooling.
The Centre of Gravity (CG), initially below the pivot axis, moves above the pivot axis, causing the bird to tilt until its beak dips into the glass of water.
This warms up the beak, which forces the red liquid back into the body. The CG then moves to below the pivot axis.
The bird returns to the vertical, and repeats the cycle as long as there is water to wet the beak.
Note that there is oscillation about the CG because of the gradual movement of liquid, negligible pivot friction and instability as the CG changes position. There is virtually no damping, which allows the oscillation to build up.
