Most nests are built during the breeding season by the male swiftlet over a period of 35 days. They take the shape of a shallow cup stuck to the cave wall. The nests are composed of interwoven strands of salivary cement. Both nests have high levels of calcium, iron, potassium, and magnesium. The nests were formerly harvested from caves, principally the enormous limestone caves at  Gomantong  and  Niah  in Borneo. With the escalation in demand these sources have been supplanted since the late- 1990s by purpose-built nesting houses, usually reinforced concrete structures following the design of the Southeast Asian shop-house ("rumah toko"/"ruko").