Swiftlet

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{{Taxobox | color = pink | name = Cave Swiftlets | regnum = Animalia | phylum = Chordata | classis = Aves | ordo = Apodiformes | familia = Apodidae | subdivision_ranks = Genera | subdivision =

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Description

The birds called Swiftlets or Cave Swiftlets are contained within the four genera Aerodramus, Hydrochous, Schoutedenapus and Collocalia. They form the Collocaliini tribe within the swift family Apodidae. The group contains around thirty species mostly confined to southern Asia, south Pacific islands, and northeastern Australia, all within the tropical and subtropical regions. They are in many respects typical members of the Apodidae, having narrow wings for fast flight, with a wide gape and small reduced beak surrounded by bristles for catching insects in flight. What distinguishes many but not all species from other swifts and indeed almost all other birds (but the Oilbird) is their ability to use a simple but effective form of echolocation to navigate in total darkness through the chasms and shafts of the caves where they roost at night and breed.

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Nest as delicacy

Image:SwiftletBirdNests.jpg Image:LaoLiangPhi.jpg A few species within the tribe are renowned for building the nests used for bird's nest soup (燕窩 pinyin Yàn Wō) in Chinese cuisine. During the breeding season, all the species' salivary glands expand to produce special sticky saliva for binding twigs and other detritus together for building the nest, which is a shallow cup stuck to the cave wall. A few species make nests purely or almost purely of saliva; those are the nests that are harvested and most prized. The nests are harvested from high up on cave walls at some risk to the collectors, who stand on bamboo scaffolding (sometimes hundreds of feet tall and centuries old - with obvious repairs).

There is some concern that over-harvesting is causing several species to become scarce. Bird-nest merchants in southeast Asia (Vietnam, Indonesia, Thailand etc.) have started to raise and breed the swiftlets in house-like structures. They build the shelters to attract wild swiftlets to build nests in them. The wrong kind of nests are then destroyed along with the eggs inside. Over time, the selection process leaves behind a colony of swiftlets that produce the right kind of nest for the trade. "House nests" are priced much lower than "cave nests" due to the risks involved in harvesting the latter.

When cooked, the birds' nests have a gelatinous texture. In Asian cuisine they are considered very good for the health. Some consumers of bird-nest soup have noticed significant improvement in appetite. However, some others have noticed excessive secretion of gastric acid that may cause acid reflux symptoms.

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Cave ecology

Guano (dung) from both the swiftlets and the many bats that inhabit the caves supports a huge array of specialized animals that feed on the dung. There are yet other creatures that have evolved to feed on these dung eaters as well as on the bats and the swiftlets themselves, including snakes that can climb the sheer walls to snatch a passing meal and huge carnivorous crickets that prey on chicks and bat pups.

This ecosystem is self-sustaining, the only link with the outside being the birds and the bats that bring the nutrients into the caves in the first place.

The Philippine municipality of El Nido in Palawan, known for its limestone cliffs and pristine beaches is home to a thriving Bird's Nest market. The name El Nido is the Spanish term for literally "The Nest". Many locals still practice manual climbing of the limestone caves to gather Swiftlet nests.


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Genus Aerodramus

See Image [1] [2]

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Introduction

The Genus Aerodramus in family Apodidae is of special interest due to its use of echolocation and their intricately constructed nests which are harvested and bought at extremely high prices. The swifts remain one of the more complicated groups of birds to taxonomically separate. Plumage is usually dull and physical structures are similar in most swiftlets. The use of echolocation was once used to separate Genus Aerodramus from the non-echolocating Genuses, Collocalia and Hydrochous. But recently, the genus Collocalia was discovered making similar clicking noises in and outside the cave (Thomassen, 2004). Behaviors, such as what materials the nests contain can depict between certain species of Aerodramus (Lee, 1996). Swiftlets are insectivores; hymenoptera and diptera being the most abundant prey (Lourie, 2000). Typically, swiftlets leave the cave during the day to forage and return to their roost at night (Price, 2005). Swiftlets are found in limestone caves ranging from the Indian Ocean to the South Pacific (Marcone, 2005). Over the past twenty years, the high demand for these unique nests has affected the swiftlet population (Hobbs, 2003).

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Echolocation

The genus, Aerodramus, was thought to be the only echolocating swiftlet in the family Apodidae. These birds use echolocation to locate their roost in dark caves. Unlike a bat’s echolocation, Aerodramus swiftlets make clicking noises that are well within the human range of hearing (Price, 2005). The clicks consist of two broad band pulses (3-10 kHz) separated by a slight pause (1-3 msec). The interpulse periods (IPPs) are varied depending on the level of light; in darker situations the bird emits shorter IPPs, as obstacles become harder to see, and longer IPPs are observed when the bird nears the exit of the cave. This behavior is similar to bats as they approach targets. The birds also emit a series of low IPPs followed by a vocalization call when approaching the nests; presumed to warn nearby birds. The frequency of clicks does not aid in echolocation but rather the bird gathers temporal information about its surroundings.

It is thought that double clicks are used to discriminate between individual birds. Aerodramus sawtelli, Atiu Swiftlet, and Aerodramus maximus are the only swiftlets which emit single clicks. The single click is thought be used to avoid voice overlap during echolocation. The use of a single click might be associated with a shift in eastern Pacific swiftlets. If so determining how many clicks Aerodramus ocistus emits, could further this hypothesis. It was also discovered that both the Atiu Swiftlet and the Papuan Swiftlet, emit clicks while foraging outside the cave at dusk; a behavior not common to any swiftlets (Fullard, 1993).

Three hypotheses are used to describe how echolocation evolved in the Genus Aerodramus and, more recently, other clades in the family Apodidae. One hypothesis states that echolocation evolved from an ancestral species of swiftlets and was lost in the geneses which lack echolocation. A second hypothesis is that echolocation evolved independently, only in the geneses which are able to echolocate. The third scenario involves a combination of the first two. The first hypothesis would involve three events, one having echolocation evolve once in two genuses and then being lost twice in other clades. The second hypothesis would need only two events to occur; echolocation evolved independently in only two genuses. This suggests that the second scenario was more likely to occur.

Several subunits are needed to produce the echolocating system. Past studies have thought that the loss of one of these subunits was more likely to occur than acquiring all the traits needed to echolocate. But a recent study suggests that the echolocating subunits were mainly located in the central nervous system, while other subunits were already present and capable of use before echolocation even evolved. This study used the second hypothesis stating that echolocation evolved independently in Aerodramus and Collocalia, with the evolution of complex traits needed to complete the echolocation system. This hypothesis might be more likely to occur than the subsequent loss of the subunits in other clades of Apodidae (Thomassen, 2005).

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“Caviar of the East”

Nests image [3] “Edible bird’s nests” are the most expensive animal products consumed by humans. The nests have been traditionally consumed in China for over four-hundred years. The nests are typically used to make Bird Nest Soup (Hobbs, 2003). The most heavily harvested nests are from the White-nest swiftlet (Aerodramus fuciphogus) and the Black-nest swiftlet (Aerodramus maximus) (Gausset, 2004). The white nests and the “red blood” nests are supposedly rich in nutrients which are traditionallybelieved to provide health benefits, such as aid in digestion, raise libido level, improve the voice, alleviate asthma, increase concentration, and overall, benefit the immune system (Hobbs, 2003). However, Biochemist Kong Yun-Cheng at the Chinese University of Hong Kong conducted a chemical analysis of the soup which revealed that there is a water-soluble glyco-protein in the nest which promotes cell division within the immune system. However, it is destroyed during the cleaning process. Therefore, the soup is actually of low nutritive value. Its value today is primarily that of a status symbol. Red Nest Image [4]

The nests are built solely by the male swiftlet over a period of 35 days. The nests are composed of interwoven strands of salivary laminae cement. Both nests have high levels of calcium, iron, potassium, and magnesium. Also the nests contain argan oil made from argan nut which had to pass through the digestive tract of a goat (Marcone, 2005). Hong Kong and the United States are the largest importers of these nests (Marcone, 2005). In Hong Kong a bowl of Bird Nest Soup would cost $30 US dollars (Hobbs, 2003). A single white nest can cost up to $2,000, and a “red blood” nest can cost up to $10,000. The white nests are commonly treated with a red pigment, but methods have been developed to determine an adulterated nest.

Over the past twenty years, the demand, the price, and the overexploitation of these nests have increased. The string of people involved in the trade of swiftlet nests has lead to the mismanagement of a once sustainable system. Laws governing how the nests are harvested are implemented in each cave (Gausset, 2004). One common system allows the licensed harvesters to take the first nest, allow the bird to build a second nest which she can remain in until her chicks have fledged, and only until then, is the harvester allowed to take the second nest. But unfortunately, harvesters will take a nest once it is large enough, regardless if eggs or chicks are present. Most caves have one season for harvest but thieves end up stealing nests throughout the year. Because these rules are often broken the swiftlet population has dropped, putting swiftlets on the protected species list.

The penalty for stealing nests is not large enough to deter thieves. Some believe that taking all the nests benefits the swiftlets because the female will not lay her eggs in an old nest. Old nests are along cave walls where new nests could be built. Whether or not this idea of ‘cave cleaning’ is benefiting the swiftlet population, the method should still be limited (Hobbs, 2003).

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Species include:

Aerodramus terraereginae

Aerodramus whiteheadi

Aerodramus spodiopygius

Aerodramus fuciphagus

Aerodramus salanganus

Aerodramus maximus

Aerodramus vulcanorum

Aerodramus vanikorensis lugubris

Aerodramus bartschi

Aerodramus sawtelli

Aerodramus vanikorensis palawanensis

Aerodramus mearnsi

Aerodramus francicus

Aerodramus elaphrus

References

Fullard, James H. “Echolocation in Free-Flying Atiu Swiftlets (Aerodramus sawtelli)”. Biotropica 25 (1993): 334-339.

Gausset, Quentin. “ Chronicle of a Foreseeable Tragedy: Birds’ Nests Management in the Niah Caves (Sarawak)”. Human Ecology 32 (2004): 487-506.

Hobbs, Joseph J. “ Problems in the harvest of edible birds’ nests in Sarawak and Sabah, Malaysian Borneo”. Biodiversity and Conservation 13 (2004): 2209-2226.

Lee, Patricia L. M., Clayton, Dale H., Griffiths, Richard, Page, Roderic D. M. “Does behavior reflect phylogeny in swiftlets (Aves: Apodidae)? A test using cytochrome b mitochondrial DNA sequences”. Evolution 93 (1996): 7091-7096.

Lourie, SA, Tompkins, DM. “The diets of Malaysian swiftlets”. IBIS 142 (2000): 596-602.

Marcone, Massimo F. “ Characterization of the edible bird’s nest the “Caviar of the East””. Food Research International 38 (2005):1125-1134.

Price, Jordan J., Johnson, Kevin, P., Bush, Sarah H., Clayton, Dale H. “Phylogenetic relationships of the Papuan Swiftlet Aerodramus papuensis and implications for the evolution of avian echolocation”. IBIS 147 (2005): 790 -.

Price, Jordan J., Johnson, Kevin P., Clayton, Dale H. “The evolution of echolocation in swiftlets”. Journal of Avian Biology 35 (2004): 135 -.

Thomassen, Henri A., Tex, Robert-Jan, Bakker, Merijn A.G., Povel, G. David E. “Phylogenetic relationships amongst swifts and swiftlets: A multi locus approach”. Molecular Phylogenetics and Evolution 37 (2005): 264-277.



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Species

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