Most bottles have two chambers, others just have one chamber. Some bottles have even four to six chambers. The whistling bottles are made of one or more chambers, which are connected by an upper bridge handle, which is where the whistle is often contained. The lower pottery tube is for the liquid or air to flow from on chamber to the other. Whistling is accomplished by air when it exits the effigy chamber through a small hole in the chamber.
Water can also produce the whistle effect on the bottle.
When a bottle is returned to the upright position, after a portion of its liquid is poured from the tubular spot, the remaining liquid, seeking its own level, displaces the air in the effigy chamber. This produces an air stream moving directly across the whistle opening.