Edible bird's nests are bird nests created by edible-nest swiftlets using solidified saliva, which are harvested for human consumption. They are particularly prized in Chinese culture due to their rarity, and supposedly high nutritional value and exquisite flavor. Edible bird's nests are among the most expensive animal products consumed by humans, with nests being sold recently at prices up to about US$2,000 per kilogram, depending on grading. The type or grading of bird's nest depends on the type of bird as well as the diet of the bird. It differs in color from white to dark brown. The Chinese believe that it promotes good health, especially for the skin. The nests have been used in Chinese cooking for over 400 years, most often as bird's nest soup.