A drinking bird consists of two glass bulbs joined by a glass tube (the bird's neck/body). The tube extends nearly all the way into the bottom bulb, and attaches to the top bulb but does not extend into it.
The space inside the bird contains a fluid, usually colored for visibility. (This dye might fade when exposed to light, with the rate depending on the dye/color).
The fluid is typically dichloromethane (DCM), also known as methylene chloride.
Earlier versions contained trichlorofluoromethane.
Miles V. Sullivan's 1945 patent suggested ether, alcohol, carbon tetrachloride, or chloroform.
Air is removed from the apparatus during manufacture, so the space inside the body is filled by vapor evaporated from the fluid. The upper bulb has a "beak" attached which, along with the head, is covered in a felt-like material. The bird is typically decorated with paper eyes, a plastic top hat, and one or more tail feathers. The whole device pivots on a crosspiece attached to the body.