The birds' nests used for soup are built by the salangane, a type of Chinese swallow. Just before breeding the birds feed on gelatinous seaweed which makes them secrete a thick, whitish, glutinous saliva which they use to cement their nests. When the nests are soaked in water, they become transparent and gelatinous, which gives birds’ nest soup its characteristic odour and texture. Chopped poultry or vegetables are often added.