Swiftlets are

bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

s from the four

genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

Aerodramus ''Aerodramus'' is a genus of small, dark, cave-nesting birds in the swiftlet, Collocaliini tribe (biology), tribe of the Swift (bird), swift family. Its members are confined to tropical and subtropical regions in southern Asia, Oceania and north ...

'', ''

Collocalia ''Collocalia'' is a genus of Swift (bird), swifts, containing some of the smaller species termed "swiftlets". Formerly a catch-all genus for these, a number of its former members are now normally (though not by all authors) placed in ''Aerodramus ...

'', ''

Hydrochous The giant swiftlet (''Hydrochous gigas''), also known as the waterfall swift, is a species of bird in the swift family, Apodidae. It is the only member of the monotypic genus ''Hydrochous''. It is found in Malaysia, Sumatra and Java, where its na ...

'' and '' Schoutedenapus'', which form the

tribe The term tribe is used in many different contexts to refer to a category of human social group. The predominant worldwide use of the term in English is in the discipline of anthropology. The definition is contested, in part due to conflict ...

Collocaliini within the

swift Swift or SWIFT most commonly refers to: * SWIFT, an international organization facilitating transactions between banks ** SWIFT code * Swift (programming language) * Swift (bird), a family of birds It may also refer to: Organizations * SWIF ...

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Apodidae The Apodidae, or swifts, form a family of highly aerial birds. They are superficially similar to swallows, but are not closely related to any passerine species. Swifts are placed in the order Apodiformes along with hummingbirds. The treeswifts ar ...

. The group contains around thirty

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

mostly confined to southern

Asia Asia ( , ) is the largest continent in the world by both land area and population. It covers an area of more than 44 million square kilometres, about 30% of Earth's total land area and 8% of Earth's total surface area. The continent, which ...

, south

Pacific The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is bounded by the cont ...

islands, and northeastern

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

, all within the

tropical The tropics are the regions of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...

and

subtropical The subtropical zones or subtropics are geographical zone, geographical and Köppen climate classification, climate zones immediately to the Northern Hemisphere, north and Southern Hemisphere, south of the tropics. Geographically part of the Ge ...

regions. They are in many respects typical members of the Apodidae, having narrow

wing A wing is a type of fin that produces both Lift (force), lift and drag while moving through air. Wings are defined by two shape characteristics, an airfoil section and a planform (aeronautics), planform. Wing efficiency is expressed as lift-to-d ...

s for fast flight, with a wide gape and small reduced

beak The beak, bill, or rostrum is an external anatomical structure found mostly in birds, but also in turtles, non-avian dinosaurs and a few mammals. A beak is used for pecking, grasping, and holding (in probing for food, eating, manipulating and ...

surrounded by

bristle A bristle is a stiff hair or feather (natural or artificial), either on an animal, such as a pig, a plant, or on a tool such as a brush or broom. Synthetic types Synthetic materials such as nylon are also used to make bristles in items such as b ...

s for catching insects in flight. What distinguishes many but not all species from other swifts and indeed almost all other birdsThe

oilbird The oilbird (''Steatornis caripensis''), locally known as the , is a bird species found in the northern areas of South America including the Caribbean island of Trinidad. It is the only living species in the genus ''Steatornis'', the family Stea ...

is a notable exception. The presence of echolocation was formerly used to argue for a close relationship of the Apodiformes and the oilbird, but the actual situation is more complicated. ''See also'':

Caprimulgiformes Nightjars are medium-sized nocturnal or crepuscular birds in the family Caprimulgidae and order Caprimulgiformes, characterised by long wings, short legs, and very short bills. They are sometimes called bugeaters, their primary source of food ...

. is their ability to use a simple but effective form of echolocation to navigate in total darkness through the chasms and shafts of the

cave Caves or caverns are natural voids under the Earth's Planetary surface, surface. Caves often form by the weathering of rock and often extend deep underground. Exogene caves are smaller openings that extend a relatively short distance undergrou ...

s where they roost at night and breed. The nests of some species are built entirely from solidified threads of their

saliva Saliva (commonly referred as spit or drool) is an extracellular fluid produced and secreted by salivary glands in the mouth. In humans, saliva is around 99% water, plus electrolytes, mucus, white blood cells, epithelial cells (from which ...

, which are edible and thus collected for human consumption as the famous Chinese

delicacy A delicacy is a rare food item that is considered highly desirable, sophisticated, or peculiarly distinctive within a given culture or region. A delicacy may have an unusual flavor or be expensive compared to everyday foods. Delicacies va ...

, the

bird's nest soup Edible bird's nests, also known as swallow nests ( zh, c=燕窝, p=yànwō), are bird nests created from solidified saliva by edible-nest swiftlets, Indian swiftlets and other swiftlets of the genera ''Aerodramus'', '' Hydrochous'', '' Schout ...

Description and ecology

The swift family remains one of the more complicated groups of birds in

taxonomic 280px, Generalized scheme of taxonomy Taxonomy is a practice and science concerned with classification or categorization. Typically, there are two parts to it: the development of an underlying scheme of classes (a taxonomy) and the allocation ...

research, but the swiftlet tribe is a rather well-defined group. Its internal

systematics Systematics is the study of the diversification of living forms, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees (synonyms: phylogenetic trees, phylogenies). Phy ...

is confusing; the plumage is usually dull, with shades of black, brown, and gray; from their outward appearance, most species are very similar. Swiftlets have four toes, except the

Papuan swiftlet The three-toed swiftlet or Papuan swiftlet (''Aerodramus papuensis'', formerly ''Collocalia papuensis'') is a species of swift. It is found in New Guinea New Guinea (; Hiri Motu: ''Niu Gini''; , fossilized , also known as Papua or historica ...

which lacks the

hallux Toes are the digits of the foot of a tetrapod. Animal species such as cats that walk on their toes are described as being ''digitigrade''. Humans, and other animals that walk on the soles of their feet, are described as being ''plantigrade''; ...

(back toe). Their legs are very short, preventing the birds from perching, but allowing them to cling to vertical surfaces. Flight is mainly gliding due to very long primary feathers and small breast muscles. The larger ''Aerodramus'' swiftlets weigh about 14 grams and are 10 cm long. Swiftlets are insectivores;

hymenoptera Hymenoptera is a large order of insects, comprising the sawflies, wasps, bees, and ants. Over 150,000 living species of Hymenoptera have been described, in addition to over 2,000 extinct ones. Many of the species are parasitic. Females typi ...

ns and

diptera Flies are insects of the order Diptera, the name being derived from the Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advance ...

ns being the most abundant prey. Typically, they leave the cave during the day to forage and return to their roost at night. Males and females look similar; as usual in such cases, these birds are monogamous and both partners take part in caring for the nestlings. Males perform aerial displays to attract females and mating occurs at the nest. The breeding season overlaps the wet season, which corresponds to an increased insect population. Clutch size depends on the location and the food source, but it is generally not large; ''Aerodramus'' swiftlets lay 1 to 2 eggs. The eggs are a dull white color and are laid every other day. Many if not all species are colonial nesters; some build their nests in high, dark corners on cave walls. Swiftlets in temperate zones do migrate, but most ''Aerodramus'' swiftlets live in the tropical

Indo-Pacific The Indo-Pacific is a vast biogeographic region of Earth. In a narrow sense, sometimes known as the Indo-West Pacific or Indo-Pacific Asia, it comprises the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the ...

region and do not migrate. These birds usually remain in one cave or other roosting/nesting site. Some examples of caves include the Niah Caves at Niah National Park &

Gunung Mulu National Park The Gunung Mulu National Park, also known simply as the Mulu National Park is a national park in Miri Division, Sarawak, Malaysia. It is a UNESCO World Heritage Site that encompasses caves and Karst topography, karst formations in a mountainous ...

which are all located in

Sarawak Sarawak ( , ) is a States and federal territories of Malaysia, state of Malaysia. It is the largest among the 13 states, with an area almost equal to that of Peninsular Malaysia. Sarawak is located in East Malaysia in northwest Borneo, and is ...

Malaysia Malaysia is a country in Southeast Asia. Featuring the Tanjung Piai, southernmost point of continental Eurasia, it is a federation, federal constitutional monarchy consisting of States and federal territories of Malaysia, 13 states and thre ...

Borneo Borneo () is the List of islands by area, third-largest island in the world, with an area of , and population of 23,053,723 (2020 national censuses). Situated at the geographic centre of Maritime Southeast Asia, it is one of the Greater Sunda ...

. The genus ''Aerodramus'' is of special interest due to its use of echolocation and its intricately constructed

nests which in some species contain no other material such as feathers, moss or twigs and are collected, selling at extremely high prices (see

Bird's nest soup Edible bird's nests, also known as swallow nests ( zh, c=燕窝, p=yànwō), are bird nests created from solidified saliva by edible-nest swiftlets, Indian swiftlets and other swiftlets of the genera ''Aerodramus'', '' Hydrochous'', '' Schout ...

). It has been argued that the high demand for these nests could have had an adverse effect on their populations, but other authorities have shown that modern techniques of nest farming have increased the bird population. The use of echolocation was once used to separate ''Aerodramus'' from the non-echolocating genera ''Collocalia'' and ''Hydrochous'' (virtually nothing is known about ''Schoutedenapus''). But recently, the

pygmy swiftlet The pygmy swiftlet (''Collocalia troglodytes'') is a species of Swift (bird), swift in the family Apodidae. It is Endemism, endemic to the Philippines. Its natural habitat is tropical moist lowland forests. At under , it is the world's smallest ...

''Collocalia troglodytes'' was discovered making similar clicking noises in and outside its cave. Characteristics of behavior, such as what materials other than saliva the nests contain, can be used to differentiate between certain species of ''Aerodramus''.

Echolocation

The genus ''Aerodramus'' was thought to be the only echolocating swiftlets. 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. The clicks consist of two broad band pulses (3–10

kHz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose formal expression in terms of SI base uni ...

) separated by a slight pause (1–3

millisecond A millisecond (from '' milli-'' and second; symbol: ms) is a unit of time in the International System of Units equal to one thousandth (0.001 or 10−3 or 1/1000) of a second or 1000 microseconds. A millisecond is to one second, as one second i ...

s). 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 that of bats as they approach targets. The birds also emit a series of low clicks followed by a call when approaching the nests; presumably to warn nearby birds out of their way. It is thought that the double clicks are used to discriminate between individual birds. ''Aerodramus sawtelli'', the Atiu swiftlet, and ''Aerodramus maximus'', the black-nest swiftlet are the only known species 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 an evolutionary shift in eastern Pacific swiftlets; determining how many clicks the Marquesan swiftlet emits could shed light on this. It was also discovered that both the Atiu swiftlet and the

emit clicks while foraging outside at dusk; the latter possibly only in these circumstances, considering that it might not nest in caves at all. Such behavior is not known to occur in other species, but quite possibly does, given that the Papuan and Atiu swiftlets are not closely related. However, it has recently been determined that the echolocation vocalizations do not agree with evolutionary relationship between swiftlet species as suggested by

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

sequence comparison. This suggests that as in

bat Bats are flying mammals of the order Chiroptera (). With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most birds, flying with their very long spread-out ...

s, echolocation sounds, once present, adapt rapidly and independently to the particular species' acoustic environment. Three hypotheses are considered to describe how echolocation evolved in the genus ''Aerodramus'' and, as determined more recently, other taxa in the Apodidae. One hypothesis states that echolocation evolved from an ancestral species of swiftlets and was lost in the genera which lack echolocation. A second hypothesis is that echolocation evolved independently several times. The third scenario involves a combination of the first two, i.e. a gain-loss-regain scenario. Several functional subunits (like vocal muscles and brain areas) 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. Yet a recent study suggests that the echolocation subunits were mainly located in the

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain, spinal cord and retina. The CNS is so named because the brain integrates the received information and coordinates and influences the activity o ...

, while the subunits in the vocal apparatus were already present and capable of use before echolocation even evolved. This study supports the second hypothesis of independent evolution of echolocation in ''Aerodramus'' and ''Collocalia'', with the subsequent evolution of complex behavior needed to complement the physical echolocation system, or even the third approach, as the vocal apparatus-parts of the echolocation system might even be inherited from some

prehistoric Prehistory, also called pre-literary history, is the period of human history between the first known use of stone tools by hominins million years ago and the beginning of recorded history with the invention of writing systems. The use o ...

nocturnal Nocturnality is a ethology, behavior in some non-human animals characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal", versus diurnality, diurnal meaning the opposite. Nocturnal creatur ...

ancestor.

Culinary use

Authentic bird's-nest soup is made from nests of some species of swiftlet, mainly the edible-nest (or white-nest) swiftlet (''Aerodramus fuciphagus'') and the black-nest swiftlet. Instead of twigs, feathers and straw, these swiftlets make their nest only from strands of their gummy saliva, which hardens when exposed to air. Once the nests are harvested, they are cleaned and sold to restaurants. Eating swiftlet nest material is believed to help maintain skin tone, balance '' qi'' ("life energy") and reinforce the

immune system The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to bacteria, as well as Tumor immunology, cancer cells, Parasitic worm, parasitic ...

. It is also believed to strengthen the lungs and prevent coughs, improve the constitution and prolong life. The nutritional value of 100 g of dry nest includes 49.9 g of water-soluble protein (including amido nitrogen, monoamine nitrogen, non-amino nitrogen, arginine, humin, histidine, lysine and cysteine), 30.6 g carbohydrate (glycoprotein and mucin), 4.9 g iron, 2.5 g inorganic salt (including potassium, sodium, calcium, magnesium, sulfur, phosphorus, silica and other trace elements), and 1.4 g fiber (Dictionary of Traditional Chinese Medicine, The History of Chinese Medicine and the Nutrition Table). The energy contained in 100 g of swiftlet nest is 345 kcal. The nests are often served simmered in

chicken The chicken (''Gallus gallus domesticus'') is a domesticated subspecies of the red junglefowl (''Gallus gallus''), originally native to Southeast Asia. It was first domesticated around 8,000 years ago and is now one of the most common and w ...

broth. Authentic bird's-nest soup is quite popular throughout Asia. It is also extremely expensive; many western restaurants serve a less expensive version consisting of soup with noodles shaped to resemble a bird's nest.

Cave ecology

Guano Guano (Spanish from ) is the accumulated excrement of seabirds or bats. Guano is a highly effective fertiliser due to the high content of nitrogen, phosphate, and potassium, all key nutrients essential for plant growth. Guano was also, to a le ...

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

snake Snakes are elongated limbless reptiles of the suborder Serpentes (). Cladistically squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales much like other members of the group. Many species of snakes have s ...

s that can climb the sheer walls to snatch a passing meal and huge carnivorous

crickets Crickets are orthopteran insects which are related to bush crickets and more distantly, to grasshoppers. In older literature, such as Imms,Imms AD, rev. Richards OW & Davies RG (1970) ''A General Textbook of Entomology'' 9th Ed. Methuen 886 ...

that prey on chicks and bat pups. This cave fauna

ecosystem An ecosystem (or ecological system) is a system formed by Organism, organisms in interaction with their Biophysical environment, environment. The Biotic material, biotic and abiotic components are linked together through nutrient cycles and en ...

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 The Philippines, officially the Republic of the Philippines, is an Archipelagic state, archipelagic country in Southeast Asia. Located in the western Pacific Ocean, it consists of List of islands of the Philippines, 7,641 islands, with a tot ...

municipality A municipality is usually a single administrative division having municipal corporation, corporate status and powers of self-government or jurisdiction as granted by national and regional laws to which it is subordinate. The term ''municipality' ...

of El Nido in

Palawan Palawan (, ), officially the Province of Palawan (; ), is an archipelagic province of the Philippines that is located in the region of Mimaropa. It is the largest province in the country in terms of total area of . The capital and largest c ...

, known for its

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

cliffs In geography and geology, a cliff or rock face is an area of rock which has a general angle defined by the vertical, or nearly vertical. Cliffs are formed by the processes of weathering and erosion, with the effect of gravity. Cliffs are com ...

and pristine

beaches A beach is a landform alongside a body of water which consists of loose particles. The particles composing a beach are typically made from Rock (geology), rock, such as sand, gravel, shingle beach, shingle, pebbles, etc., or biological s ...

, is home to a thriving bird's-nest market. The name El Nido is the

Spanish Spanish might refer to: * Items from or related to Spain: **Spaniards are a nation and ethnic group indigenous to Spain **Spanish language, spoken in Spain and many countries in the Americas **Spanish cuisine **Spanish history **Spanish culture ...

term for literally "The Nest". Many locals still practice manual climbing of the limestone caves to gather swiftlet nests.

Species

The

is apparently closer to the waterfall swift than to the other ''Aerodramus'' species and probably best placed in a separate genus, whereas Thomassen ''et al.'' (2005) advocate reuniting all swiftlets in ''Collocalia''. ''Schoutedenapus'' is one of the least-known genera of birds. *Genus ''

'' ** Plume-toed swiftlet, ''Collocalia affinis'' ** Grey-rumped swiftlet, ''Collocalia marginata'' ** Ridgetop swiftlet, ''Collocalia isonota'' ** Tenggara swiftlet, ''Collocalia sumbawae'' ** Drab swiftlet, ''Collocalia neglecta'' ** Glossy swiftlet, ''Collocalia esculenta'' ** Satin swiftlet, ''Collocalia uropygialis'' ** Bornean swiftlet, ''Collocalia dodgei'' ** Cave swiftlet, ''Collocalia linchi'' ** Christmas Island swiftlet, ''Collocalia natalis'' **

Pygmy swiftlet The pygmy swiftlet (''Collocalia troglodytes'') is a species of Swift (bird), swift in the family Apodidae. It is Endemism, endemic to the Philippines. Its natural habitat is tropical moist lowland forests. At under , it is the world's smallest ...

, ''Collocalia troglodytes'' *Genus ''

'' ** Seychelles swiftlet, ''Aerodramus elaphrus'' ** Mascarene swiftlet, ''Aerodramus francicus'' ** Indian swiftlet, ''Aerodramus unicolor'' ** Philippine swiftlet, ''Aerodramus mearnsi'' ** Moluccan swiftlet, ''Aerodramus infuscatus'' ** Mountain swiftlet, ''Aerodramus hirundinaceus'' ** White-rumped swiftlet, ''Aerodramus spodiopygius'' ** Australian swiftlet, ''Aerodramus terraereginae'' **

Himalayan swiftlet The Himalayan swiftlet (''Aerodramus brevirostris'') is a small swift. It is a common colonial breeder in the Himalayas and Southeast Asia. Some populations are migratory. This swiftlet was formerly placed in the genus ''Collocalia''. Two of i ...

, ''Aerodramus brevirostris'' ** Indochinese swiftlet, ''Aerodramus rogersi'' ** Volcano swiftlet, ''Aerodramus vulcanorum'' ** Whitehead's swiftlet, ''Aerodramus whiteheadi'' ** Bare-legged swiftlet, ''Aerodramus nuditarsus'' ** Mayr's swiftlet, ''Aerodramus orientalis'' ** Palawan swiftlet, ''Aerodramus palawanensis'' **

Mossy-nest swiftlet The mossy-nest swiftlet (''Aerodramus salangana'') is a species of swift in the family Apodidae. Some taxonomists consider it to be a subspecies of the uniform swiftlet. It is found in northern Borneo, the Natuna and the Derawan Islands and ...

, ''Aerodramus salangana'' ** Uniform swiftlet, ''Aerodramus vanikorensis'' ** Palau swiftlet, ''Aerodramus pelewensis'' ** Guam swiftlet, ''Aerodramus bartschi'' ** Caroline Islands swiftlet, ''Aerodramus inquietus'' ** Mangaia swiftlet, ''Aerodramus manuoi'' (

) ** Atiu swiftlet, ''Aerodramus sawtelli'' ** Polynesian swiftlet, ''Aerodramus leucophaeus'' ** Marquesan swiftlet, ''Aerodramus ocistus'' ** Black-nest swiftlet, ''Aerodramus maximus'' **

Edible-nest swiftlet The edible-nest swiftlet (''Aerodramus fuciphagus''), also known as the white-nest swiftlet, is a small bird of the swift family which is found in Southeast Asia. Its opaque and whitish bird nest is made exclusively of solidified saliva and is t ...

, ''Aerodramus fuciphagus'' *** Brown-rumped swiftlet, ''Aerodramus (fuciphagus) vestitus'' ** German's swiftlet, ''Aerodramus germani'' **

, ''Aerodramus papuensis'' – probably a distinct genus *Genus ''

'' ** Giant swiftlet, ''Hydrochous gigas'' *Genus '' Schoutedenapus'' ** Scarce swift, ''Schoutedenapus myoptilus''

Description and ecology

Echolocation

Culinary use

Cave ecology

Species

References

Further reading