How do swiftlets use saliva to build nests?

Swiftlets, which belong to the family Apodidae, are unique for their ability to construct nests primarily or entirely out of their own saliva. During the nesting process, these birds produce a glutinous secretion from their salivary glands that dries to form a firm, bracket-shaped structure.12

While these nests may occasionally incorporate small amounts of materials like bark or fern, they are typically attached to the walls of coastal or mountain caves. This hardened salivary secretion is the primary ingredient used in bird’s-nest soup. In addition to this remarkable building behavior, certain species of swiftlets are notable for using echolocation to navigate the dark environments where they build their colonies.12