What is birds nest soup made from?

Birds' nest soup is a delicacy made from the nests of edible-nest swiftlets, which are found in Southeast Asia. The nests are made from the swiftlets' saliva and are considered a delicacy in Chinese cuisine. The soup is made by simmering the nests in water until they dissolve, and then adding various other ingredients such as chicken, pork, and vegetables. The soup is often served with rice and is considered a very expensive and luxurious dish.