... is most commonly made from nests of either the edible–nest swiftlet or the black–nest swiftlet. Both are members of the swift family (so no problem with the question – or the answer).
The nests are made using solidified saliva. They are prized in Chinese culture due to their rarity, and supposedly high nutritional value and exquisite flavour. The Chinese believe that they promote good health, especially for the skin.
Edible birds' nests are among the most expensive animal products consumed by humans, with an average nest retailing for about US$2,500 per kilogram. (Wikipedia doesn't tell us how much the average nest actually weighs – but I imagine it's nothing like a kilogram.)
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