Bird saliva is very good for making glue: in fact, some species of swiftlets make their whole nest out of simply saliva. The male regurgitates (throws up) a long, thin strand of saliva from glands under its tongue. This saliva is used to be made into a half-cup nest which sticks to a cave wall (swiftlets like to build their nests in caves and grottos) and dries quickly. This saliva nest is thought to be a delicacy (something "really" yummy to eat) in China, and it is very expensive. They put it into a soup called Bird's Nest Soup.