They often feed in large flocks with other species of swiftlet and swallow. It breeds in colonies in coastal areas, in limestone caves, in rock crevices, in a cleft in a cliff or sometimes on a building. The bracket-shaped nest is built on a vertical surface and the long legs are used for clinging. These swifts never settle voluntarily on the ground. The nest is white and translucent and is made of layers of hardened saliva attached to the rock. It measures about 6 cm across with a depth of 1.5 cm and a weight of about 14 grams. Two white, oval, non-glossy eggs are laid. At breeding colonies, the bird emits high-pitched and burbling calls. They also emit a rattling call used for echolocation, which enables them to look for their nesting sites in the darkness of caves. Threats and conservation The nest used in bird's nest soup is composed entirely of saliva. The soup is made by soaking and steaming the nests in water. It is said to improve kidney health, reduce phlegm, and to be an aphrodisiac. The nests can fetch high prices and many colonies are harvested commercially. Some populations such as those in the Andaman and Nicobar Islands have been harvested extensively leading to them being considered critically threatened under the IUCN criteria. The use of artificial bird houses is growing. A detailed account of modern nest farming and marketing techniques is given by David Jordan (2004). In Indonesia and Malaysia, "farming" of nests is performed in purpose-built structures or old empty houses with "tweeters" playing recordings of bird calls on the roof to attract swiftlets. In urban areas, such "bird houses" may be considered a nuisance by neighbours due to the loud bird calls and bird feces. References Further reading MacKinnon, John & Phillipps, Karen (1993) A Field Guide to the Birds of Borneo, Sumatra, Java and Bali, Oxford University Press, Oxford. Manchi, Shirish S.; Sankaran, Ravi (2010). Foraging habits and habitat use by edible nest and glossy swiftlets in the Andsman Islands The Wilson Journal of Ornithology. 122 (2): 259–272. ISSN 1559-4491. Robson, Craig (2002) A Field Guide to the Birds of South-East Asia, New Holland Publishers (UK) Ltd., London. External links Oriental Bird Images: Edible-nest Swiftlet Review of Scientific Research on Edible Bird's Nest edible-nest swiftlet Birds of Southeast Asia Cave birds edible-nest swiftlet