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Would you eat soup made from petrified bird saliva?

By Steven Law, KSL.com Contributor  |  Posted Jul 5th, 2013 @ 11:42am


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SALT LAKE CITY — It’s called bird’s nest soup in China, where it’s very popular. It's often called the “caviar of the East” but given its somewhat rubbery, somewhat viscous texture, it should perhaps be called the “escargot of the East.”

Birds nest soup is made from the actual nest of a few particular types of swiftlets. But don’t picture, as most do, a nest made of twigs, grass and mud — the nest of this swiftlet is composed entirely from the swiftlet’s saliva.

When building its nest the, swiftlet spools out a string of its saliva, which has a glue-like, stringy consistency. Imagine an al dente angel hair pasta noodle and you’ve got the idea. The saliva is a mucoglycoprotein (a protein coupled with a carbohydrate) and it contains high levels of calcium, iron, magnesium and potassium, which, then hardens into a substance biologists call salivary laminae cement.

The nests are built by both the male and female swiftlets, and they raise their young together, according to Jeremy Shipley, a Ph.D. student at Cornell University who studies swiftlets and their close relatives.

The saliva is produced by an oversized pair of salivary glands under the bird’s tongue, which enlarges during the mating and nest-building season, and shrinks when nesting season is finished, Shipley said. TIt takes 35 days to complete.

Swiftlet nests before being cooked and eaten.

“It has been theorized that edible-nest swiftlets and some of their close relatives, build their nests in hard-to-reach places, such as cliffs and caves, to minimize the risk of predation on their young," Shipley said.

“Even though the edible-nest swiftlets are well-known for their ability to make nests entirely out of saliva, this trait is not unique to them,” he said. “Many other species—even some found in the U.S., such as the Chimney Swift—have the ability to create a sticky saliva that they use to fasten their nest-building materials to a vertical surface.”

According to Kang Nee, and Lee Phen Guan, both zoologists at the National University of Singapore, it is believed that edible-nest swiftlets ability grew from the reality that they were building nests on cliffs far out to sea where other nest-building materials were scarce, or too far away to efficiently gather.

There are about 30 species of swiftlets, primarily found in southern Asia, south Pacific islands, and northeastern Australia. Many species of swiftlet, but not all, use a simple but effective form of echolocation, which they employ to help them navigate the darker recesses of the caves where they build their homes. Swiftlets are aerial insectivores, meaning they feed on flying insects they catch on the wing, according to Nee and Guan.

It’s the nests of the White-nest swiftlet, Aerodramus fuciphagus, and the Black-nest swiftlet, Aerodramus maximus, that get harvested the most.


The soup is made by soaking or steaming the nest in water, and served after it has dissolved into a gelatinous texture.

The soup is made by soaking or steaming the nest in water, and served after it has dissolved into a gelatinous texture.

Turning a nest into soup, which, when dry has a consistency of a petrified sponge, is a long process. First the nest is soaked in water until it absorbs the water and expands. This can take up to 48 hours. The next step is to remove any impurities from the nest, usually feathers and droppings which accumulate while the swiftlet is building its nest. Finally the nest is placed on a strainer and rinsed with clean water.

The strands of bird’s nest soup are themselves flavorless. They add a unique bulk and texture to the soup. The soup can then be flavored with whatever the chef desires, anything from crab meat, ham, chicken, or shark’s fin broth. The fibers of the birds’ nest are also commonly made into custard, or a warm, sugary dessert soup, called tong-sui. The reconstituted bird’s nests, when made into soup, is both slimy, and rubbery. When the strands of the nest are soaked and heated to liquid it is a very slimy, viscous liquid. But the more resistant strands of swiftlet saliva that don’t reach a liquid state retain a rubbery, calamari-like texture.

It’s an acquired taste.

But bird’s nest soup isn’t eaten for its flavor; it’s eaten for its health benefits. Birds nest soup has been considered a delicacy in China since at least the Tang Dynasty (618-907). For centuries traditional Chinese medical doctors have been prescribing birds nest soup to treat asthma, bronchitis, to rejuvenate skin, and to boost energy, writes Massimo Marcone in Food Research International.

A swiftlet in flight.

Birds nest soup was prized among the affluent class as a health-giving tonic. It was believed to stimulate cell growth, balance life energy, and strengthen the lungs and immune system. It is credited with giving strong children to pregnant women, erasing wrinkles for old ladies and providing immunity to diseases for children according to Marcone.

The very best nests were reserved for the emperors and empresses, who ate them with the reserve of a holy sacrament. And birds nest soup was (and still is) considered an aphrodisiac in China. Bolstered by all this history, lore, and legend bird’s nest soup has become very popular. A bowl of bird’s nest soup cost’s between $40 and $60 in America, and sells for more than $2,000 USD per kilogram.

This spells trouble for the swiftlets.

Trade in swiftlet bird nests is a multi-million dollar industry in Asia, with more than 100 tons of bird nests consumed each year in Hong Kong alone, writes Theresa Park, in Koreabridge Writings.

Traditionally the nests were harvested from limestone caves and cliff walls in offshore islands North Borneo, Indonesia, Malaysia, Thailand, Vietnam and the Phillipines. But now, due to their rise in popularity, swiftlets are raised domestically for the sole purpose of harvesting their nests.

Traditional nest harvesting from caves and cliff faces is still common, and it’s a dangerous job, unregulated by any official body. Harvesters typically climb trellises or scaffolds built of bamboo to reach the nests, writes Park. Nest gatherers use a specialized, three pronged tool, called a rada, to delicately pry the nests from the walls. Peak nest gathering season is between February and May. An individual harvester can collect as many as 60 nests in a day.

After a nest is harvested the swiftlets will rebuild it again in the same spot, and the nest harvesters will return and remove it again. Typically the swiftlet will make a third nest, and this one some harvesters will let remain until the swiftlets have raised their young. But with nests reaching exhorbitant prices of $2,000 USD per kilogram, and higher, in China, snatching a nest that still contains eggs or young swiftlets proves too tempting for some nest gatherers who tear the nest from the cliff, dumping the eggs or chicks.

With these nests selling for such ludicrously high prices, entrepreneurs were quick to take notice. Businessmen in Thailand, Burma and Indonesia have built swiftlet condos, to attract and accommodate white swiftlets. Gray, industrial and strictly utilitarian, these swiftlet condos bear a close resemblance to an inner-city housing project.

In the swiftlet condos, harvesters gather the first two rounds of nests, allow the third series of nests to remain until they’ve raised their young, then harvest the third nest after the swiftlet chicks have left the nest.

If you have a science subject you'd like Steven Law to explore in a future article send him your idea at curious_things@hotmail.com.

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