Why is the drinking bird not an example of perpetual motion system?

Such drinking birds are referred to as insatiable birdies. Sometimes they are called dunking birds, also drinky birds or even dipping birds. These are some kind of conceptual heat engines that imitate the movements of a bird drinking from some kind of water source. Sometimes these are incorrectly referred to as samples of perpetual motion devices.

This product operates when water evaporates resulting from the fuzz on the dippy bird's head, then when the head is cooled, the temperature goes down in the head and condenses the methylene chloride vapor. This decreases the vapor pressure in the head and relative to the vapor pressure which is in the abdomen.