The Chinese prize these small, cup-shaped bird’s nests, which are used to make sweet and savory soups, as highly as they do shark’s fins. The homes of tiny swifts that migrate annually to the islands and cliffs of Southeast Asia, the nests are the dried spittle the birds use to weave their abodes from twigs, leaves, and feathers. For more than half a millennium, the Chinese have treasured these rare nests, believing they extend life, strengthen the immune system, encourage growth, and preserve youthful complexions.