This is a taxonomically difficult group of very similar species. Echolocation, DNA sequencing and parasiticlice have all been used to establish relationships, but some problems, such as the placement of the Papuan swiftlet are not fully resolved. These swiftlets can pose major identification problems where several species occur.
What distinguishes Aerodramusswiftlets from other swifts, and indeed almost all other birds, is their ability to use a simple but effective form of echolocation. This enables them to navigate within the breeding and roosting caves.
The nests of Aerodramus swiftlets are constructed with saliva as a major component. In two species, saliva is the only material used, and the nests are collected for the famous Chinese delicacy 'bird's nest soup', the over-collection of which puts pressure on the swiftlet populations.
Aerodramus swiftlets are in many respects typical swifts, having narrow wings for fast flight, and a wide gape and small reduced beak surrounded by bristles for catching insects in flight. They have dull plumage which is mainly in shades of black, brown, and grey. Members of this genus typically have dark brown upper wings and upper body, sometimes with a paler rump, light brown underparts, a paler throat, and brownish-white under-wings with dark brown "armpits". Males and female plumages are similar in appearance, as is that of the juvenile, for those species for which it has been described; in some species the juvenile shows pale fringes to the flight feathers.[3]
The legs, as with many swifts, are very short, preventing the birds from perching, but allowing them to cling to vertical surfaces. The flight is mainly gliding due to very long primary feathers and small breast muscles. Aerodramus swiftlets, depending on species, weigh 8–35grammes (0.28–1.23oz) and are 9–16centimetres (0.28–1.23in) long. These swiftlets are very similar, and where several species occur, such as Borneo, New Guinea and the Philippines, may not be separable in the field.[3]
Behaviour
These juvenile Germain's swiftlets have fallen from nests, and are being reared by hand.
Aerodramus swiftlets are aerial insectivores, which take prey like flies on the wing. They roost and breed in caves; during the day they leave the caves to forage for food, and return to roost at night. They 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.[3]
Clutch size depends on the location and the food source, but generally Aerodramus swiftlets lay one or two eggs.[3] The eggs are a dull white, 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.[3]
Characteristics of behaviour, such as what materials apart from saliva the nests contain, can be used to differentiate between certain species of Aerodramus.[9]
The genus Aerodramus is of special interest due to its use of echolocation. The swiftlets use this technique to navigate in darkness through the chasms and shafts of the caves where they breed and roost at night. Apart from swiftlets, the only other avian species to use echolocation is the unrelated oilbird.[10][11]
The Aerodramus swiftlets' echolocating double clicks are within the normal human hearing range and up to 3 milliseconds apart, with the interval becoming shorter in darker locations. Unlike the rest of the genus (for those species which have been studied), the Atiu swiftlet, Aerodramus sawtelli, and the black-nest swiftlet, A. maximus, emit only single clicks. The former species also uses echolocation outside its caves.[12]
The use of echolocation was once used to separate Aerodramus from the other non-echolocating cave swiftlet genera Collocalia and Hydrochous (virtually nothing is known about Schoutedenapus). However, recently, the pygmy swiftlet, Collocalia troglodytes, was discovered making similar clicking noises both inside and outside its roosting cave.[13]
It has recently been determined that the echolocation vocalizations do not agree with evolutionary relationship between swiftlet species as suggested by DNA sequence comparison.[14] This suggests that as in bats, echolocation sounds, once present, adapt rapidly and independently to the particular species' acoustic environment.
A study[15] suggested that the echolocation subunits were mainly located in the central nervous system, while the subunits in the vocal apparatus were already present and capable of use before echolocation even evolved. This study supports the hypothesis of independent evolution of echolocation in Aerodramus and Collocalia, with the subsequent evolution of complex behaviour needed to complement the physical echolocation system, or just possibly that the vocal apparatus-parts of the echolocation system might even be inherited from some prehistoricnocturnal ancestor.
It has been suggested that the giant or waterfall swiftlet, Hydrochous gigas, which cannot echolocate, may be descended from an echolocating ancestor.[3]
Saliva nests
An edible nest
The intricately constructed saliva nests of this swiftlet genus, which in some species contain no other material, are collected to make the delicacy bird's nest soup. They therefore command extremely high prices.
Authentic bird's nest soup is made from the nests of the edible-nest swiftlet (or white-nest swiftlet), Aerodramus fuciphagus, and the black-nest swiftlet, Aerodramus maximus. Instead of incorporating twigs, feathers and straw like others in the genus, these two swiftlets make their nest only from strands of their gummy saliva, which harden when exposed to air. Once the nests are harvested, they are cleaned and sold to restaurants. Over the past twenty years, the high demand for the nests of these Aerodramus species has had an adverse effect on their populations.[16][17] The Niah caves population of black-nest swiftlets plunged from around 1.5 million pairs in 1959 to 150,000–298,000 pairs in the early 1990s through over-harvesting.[3]
Early authors had doubts about the material used to make the nest, with whale and fish sperm and sea foam being proposed as the basis for construction. Even in the 1830s, when the use of saliva had been fairly well established, it was believed that it was only a cement to bind a sea plant which provided the bulk of the gelatinous material of the nest.[18]
Lice
As with other taxonomically difficult groups, ectoparasites can give information on relationships.[19] A study of swiftlet parasites in northern Borneo involved transferring lice between closely related swiftlet species.[20] The survival of lice in most of these transfers was significantly reduced in proportion to the mean difference in feather barb size between the donor and recipient species of hosts. Thus, adaptation to a particular resource on the body of the host appears to govern the specificity of swiftlet lice. In transfers where lice survived, the lice moved to different areas on the body of the host where the mean barb diameter of the feathers on which the lice occurred had the required value.
Papuan swiftlet
The Papuan swiftlet, Aerodramus papuensis, has three toes instead of the usual four in this group. It has the ability to echolocate, but whereas other previously studied species use echolocation primarily while flying in their caves, the Papuan swiftlet appears to be nocturnal or crepuscular and uses echolocation while active outside at night. It uses single, not double, clicks.[3][21] DNA sequence data provides strong support for a basal relationship between A. papuensis and other Aerodramus taxa and suggest that this species and the waterfall swiftHydrochous gigas, are sister taxa, a relationship that would indicate paraphyly of the genus Aerodramus.[21]
Taxonomy
The genus Aerodramus was introduced (as a subgenus of Collocalia) in 1906 by the American ornithologist Harry C. Oberholser. He designated the type species as Collocalia innominataHume. This taxon is now considered as a subspecies of the Himalayan swiftlet (Aerodramus brevirostris).[22][23] The genus name combines the Ancient Greekaēr meaning "air" with -dromos meaning "-racer" (from trekhō "to run").[24]
The swifts are a family, Apodidae, 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 with hummingbirds. The treeswifts are closely related to the true swifts, but form a separate family, the Hemiprocnidae.
Swiftlets are birds from the four genera Aerodramus, Collocalia, Hydrochous and Schoutedenapus, which form the tribe Collocaliini 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 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.
The uniform swiftlet,, also known as the Vanikoro swiftlet or lowland swiftlet, is a gregarious, medium-sized swiftlet with a shallowly forked tail. The colouring is dark grey-brown, darker on the upperparts with somewhat paler underparts, especially on chin and throat. This species is widespread from the Philippines through Wallacea, New Guinea and Melanesia. It forages for flying insects primarily in lowland forests and open areas. It nests in caves where it uses its sense of echolocation, rare in birds, to navigate.
The Himalayan swiftlet is a small swift. It is a common colonial breeder in the Himalayas and Southeast Asia. Some populations are migratory.
The Seychelles swiftlet is a small bird of the swift family. It is found only in the Seychelles Islands in the Indian Ocean.
The edible-nest swiftlet, 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 the main ingredient of bird's nest soup, a delicacy of Chinese cuisine.
The Mariana swiftlet or Guam swiftlet is a species of swiftlet in the family Apodidae.
Germain's swiftlet is a species of swift.
The mountain swiftlet is a species of swift in the family Apodidae. It is endemic to the island of New Guinea and the nearby islands of Karkar, Yapen and Goodenough. It was once placed in the genus Collocalia but has been moved, with many others, to Aerodramus. The species is divided into three subspecies, with the nominate, A. h. hirundinacea ranging over most of New Guinea, the subspecies A. h. excelsus occurring over 1600 m in the Snow Mountains and Cartenz peaks of Irian Jaya and A. h. baru being restricted to Yapen Island. It occurs in alpine areas from 500 m to the treeline. Its natural habitat is tropical moist montane forests and other mountainous habitats in New Guinea. It also occurs in lower numbers in the lowlands near hills.
The Tahiti swiftlet or Polynesian swiftlet is a species of swift in the family Apodidae. While often compared to the Marquesan Swiftlet, this bird is often more pale faced. The subspecies is the Monotypic which means it does not include a subspecies.
The three-toed swiftlet or Papuan swiftlet is a species of swift. It is found in New Guinea.
The Atiu swiftlet or Sawtell's Swiftlet is a species of bird in the swift family, endemic to Atiu in the Cook Islands.
The pygmy swiftlet is a species of swift in the family Apodidae. It is endemic to the Philippines.
The giant swiftlet, 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 natural habitats are subtropical or tropical moist montane forests and rivers. It is threatened by habitat loss.
The Australian swiftlet is a small bird belonging to the genus Aerodramus in the swift family, Apodidae. It is endemic to Queensland in north-eastern Australia. It was formerly included in the white-rumped swiftlet but is now commonly treated as a separate species. It has two subspecies which are occasionally regarded as two separate species: A. t. terraereginae and A. t. chillagoensis.
The Mangaia swiftlet is an extinct species of bird in the swift family. It became extinct during prehistoric times. It was endemic to Mangaia in the Cook Islands. It was closely allied with the extant Atiu swiftlet of Atiu, Mangaia's neighbouring island, though it was probably slightly larger.
The Christmas Island swiftlet, also known as the Christmas glossy swiftlet or the Christmas cave swiftlet, is a small bird in the swift family Apodidae. It is endemic to Christmas Island, an Australian territory in the eastern Indian Ocean. It was formerly commonly treated as a subspecies of the glossy swiftlet.
The Apodinae are a subfamily of swifts and contain the following species:
The plume-toed swiftlet is a small bird in the swift family Apodidae. It is found on some eastern Indian Ocean islands, the Malay Peninsula, Sumatra, and lowland Borneo.
1 2 3 4 5 6 7 8 9 Chantler, P.; Driessens, G. (2000). Swifts: a Guide to the Swifts and Treeswifts of the World. Mountfield, East Sussex: Pica Press. ISBN978-1-873403-83-9.
↑ Thomassen, H.A.; den Tex, R.-J.; de Bakker, M.A.G.; Povel, G.D.E. (2005). "Phylogenetic relationships amongst swifts and swiftlets: A multi locus approach". Molecular Phylogenetics and Evolution. 37 (1): 264–277. doi:10.1016/j.ympev.2005.05.010. PMID16006151.
↑ Hobbs, J.J. (2004). "Problems in the harvest of edible birds' nests in Sarawak and Sabah, Malaysian Borneo". Biodiversity and Conservation. 13 (12): 2209–26. doi:10.1023/B:BIOC.0000047905.79709.7f. S2CID34483704.
↑ Marcone, M.F. (2005). "Characterization of the edible bird's nest the Caviar of the East". Food Research International. 38 (10): 1125–34. doi:10.1016/j.foodres.2005.02.008.
↑ Page, R. D. M., Lee, P. L. M., Becher, S.A., Griffiths, R., Clayton D. H. (1997). "A Different Tempo of Evolution in Birds and their Parasitic Lice" Text retrieved 12 Nov 2007Archived 28 May 2008 at the Wayback Machine
↑ Tompkins, D.M.; Clayton D.H. (1999). "Host resources govern the specificity of swiftlet lice: size matters". Journal of Animal Ecology. 68 (3): 489–500. doi:10.1046/j.1365-2656.1999.00297.x.
1 2 Price J.J.; Johnson, K.P.; Bush, S.E.; Clayton, D.H. (October 2005). "Phylogenetic relationships of the Papuan Swiftlet Aerodramus papuensis and implications for the evolution of avian echolocation". Ibis. 147 (4): 790–6. doi:10.1111/j.1474-919X.2005.00467.x. S2CID24433492.
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