The bulb of the bird is filled with dichloromethane $CH_2Cl_2$, a colorless liquid, colored here for better visibility.
The highly volatile dichloromethane has $ t_{eb} $ $=$ $ 40^oC $
- 1 - The beak is humidified.
Because of the slope, the "drinking straw" gets out of the dichloromethane liquid in the bulb, which explains the ebb in the straw.
- 2 - The water in the humidified beak evaporates.
As evaporation is an endothermic transformation, the temperature of the head of the bird decreases.
As a result of the law of Charles
$\frac {P_1}{P_2} = \frac{T_1}{T_2} $ (special case of the ideal gas law if the volume is constant),
the pressure must be reduced in the head of the bird.
But then the dichloromethane is drawn into the straw. The vapor pressure above dichloromethane must be balanced by
a) the pressure of the liquid column and
b) the lower pressure remaining in the head.
- 3 - If the liquid column exceeds enough the center of gravity, the bird must bow again to the water tank and immerse its beak.
