Once the bird dips forward, fluid moves back into the abdomen, causing the bird to become bottom-heavy and tip up. The temperature decrease in the head condenses the methylene chloride vapor, decreasing the vapor pressure in the head relative to the vapor pressure in the abdomen. The greater vapor pressure in the abdomen forces fluid up through the neck and into the head. As fluid enters the head, it makes the Dippy Bird top-heavy. The bird tips.
Liquid travels to the head.
The bottom of the tube is no longer submerged in liquid. Vapor bubbles travel through the tube and into the head. Liquid drains from the head, displaced by the bubbles. Fluid drains back into the abdomen, making the bird bottom-heavy. The bird tips back up.