A few species of swift, namely cave swifts, are renowned for building the nests which are an incredible expensive and tasty product. The nests have been traditionally consumed in Asia, most often as Bird's Nest Soup. When dissolved in water, the birds' nests have a gelatinous texture. The nests are composed of interwoven strands of salivary laminate cement. Both nests have high levels of calcium, iron, potassium, and magnesium

More about birds nest as a product in this wiki site:

Here is a basic recipe: http://www.recipecottage.com/asian/birds-nest-soup.html