Tribe Collocaliini consists of at least 24 species across three genera, although this is an area of substantial disagreement. Swiftlet phylogeny has always been particularly challenging due to a lack of major physical characteristics by which species can be distinguished. Recent molecular analyses has thrown the taxonomy into debate, and the total number of species and genera has shifted over time. All agree, however, that Tribe Collocaliini is monophyletic within Family Apodidae. The swiftlets are united as a group of minute acrobatic aerial insectivores and are largely distinguished by unusual nest site selection within caves and waterfalls. Most notably 'one genus Aerodramus' has the ability to use echolocation which is highly unusual for an avian taxa (Thomassen 2005).
The Swiftlets are primarily a tropical and subtropical Tribe, with members' ranges spanning from the West of India to the remote Pacific Islands, with representation as far north as central China and as far south as Australia. Swiftlets may be found at most elevations with one representative found in the Himalayan mountains and several others found at sea level (Medway 1966). Many of the thirty species are restricted to a single island or island chain, although continental species may have more wide-ranging distributions. While most of the species are tropical/subtropical residents, those species in more temperate zones do migrate (Waugh and Hails 1993).
Swiftlets are perhaps most notable for their unique and stringent habitat requirements. All extant members of the Tribe roost exclusively in caves or cave-like habitats; such as in craters or behind waterfalls (Thomassen 2005). Here, swiftlets typically form large colonies, often coexisting with bats. Barring the 'oilbirds Steatornis caripensis', other avian taxa do not live in caves due to the challenges implicit in safely navigating dark caverns while flying. In swiftlets, this hurdle is overcome by the widespread use of echolocation to navigate dark caverns. Outside of their roosting sites, swiftlets are more flexible in their habitat demands. Their range encompasses tropical rain forests, temperate forests, mountains, and remote tropical islands. They are restricted only by the presence of aerial insects, as they rarely if ever land for an extended period of time. Swiftlet ranges do tend to be relatively disjointed, with populations often separated by hundreds of miles between large roosts (Medley 1966).
Swiftlet phylogeny is notoriously challenging and has been modified several times in recent years. This is largely due to a lack of morphological characters to assess as characters and the disparate range of the species in the Tribe. Swiftlets were originally placed in the single genus Collocalia but were later separated on the basis of echolocation, with Aerodramus containing those species which were capable of the behavior and Collocalia containing those which could not. A third monotypic genus Hydrochous was created for the substantially larger giant swiftlet. A fourth genus, Schoudetenapus, contains two species but next to nothing is known of either of them.
With the introduction of genetic data and new discoveries pertaining to echolocation in swiftlets, some have called for the swiftlets to be unified in a single genus once again (Thomassen et al 2003). As pygmy swiftlets appear capable of echolocation the character may no longer be used as an identifying characteristic for Aerodramus. Additionally, Hydrochous gigas appears to be entrenched in the genus Aerodramus further rendering echolocation useless as a defining character.
Despite this confusion it does appear that the swiftlets are fully monophyletic within Apodidae and that Tribe Collacaliini is a valid taxon. The question remains as to whether it would be better suited as a genus containing all species of swiftlet.
Swiftlets are diminutive and indistinct birds with few notable morphological characteristics. All members of Tribe Collocaliini are dull in physical appearance with plumage ranging from dark brown to black, although some species have limited streaking of white or gray (Medway 1966). As with all swifts, swiftlets are nearly exclusively observed in flight. In form they are compact and rounded with little distinction between head and body. The wings are held straight outwards, with a strongly curved leading edge tapering to a sharp point where it meets the straight trailing edge. The beak is short but opens wide laterally and is fringed with rictal bristles to enable easier capture of insects in flight. There is some degree of variation in size across the tribe. The smallest representatives, pygmy swiftlets average 5.4 grams (Sanchez 2014) while members of the largest species, giant swiftlets, average 35 grams (Somadikarta 1968). Swifts do not exhibit sexual dimorphism beyond minor variation in size and swiftlets are no exception (Brooke 1971b). Due to these similarities and the difficulties implicit in observing fast aerial insectivores, visual identification of swiftlets is nearly impossible in many cases. Behavioral characteristics and locality are typically used for field identification (Medway 1966).
Swiftlets are long-lived monogamous birds (Aowphol and Voris 2008). Little is known about their courtship behavior, although other aspects of their breeding behavior including nesting are well described and discussed in later sections.
Nest site selection and construction are among the most notable characteristics of Tribe Collocaliini. All species nest in caves or behind waterfalls, areas which are rarely ever inhabited by other avian taxa. The construction of the nests themselves is unique in that saliva is used as a building material to some extent by all swiftlets. Some species incorporate other materials, but others build their nests exclusively of saliva. These form the basis for the Chinese delicacy “bird’s nest soup.” All nests take the form of a ‘bracket’ on a vertical wall (Lack 1956). Depending on species, either one or two eggs are incubated each year (Lack 1956). This can occur anytime between August and April. Incubation lasts 21 to 28 days, dependent on species. Following this, it takes between 38 and 60 days for the altricial young to fledge. Breeding success can be fairly high in some species, with survival rates over 50% found in some studies (Tarburton 1986).
Across Apodidae both males and females provide parental care. During incubation parents trade off brooding every 24 hours. Feeding is also shared by parents. Eggs from multiple parents have been found in the same nest. The exact motive behind this behavior is unclear. it could either be a case of "egg-dumping" or nest parasitism induced by desperation over nest failure, or extra-parental cooperation. Given that chimney swifts are known to engage in cooperative chick rearing and the similarly colonial nature of swiftlet nesting, both of these alternatives are possible (Tarburton 1986).
Swiftlets typically live between 15 and 18 years. This is largely known from the bird’s nest industry where birds return to the same site each year, as swiftlets are rarely if ever kept in captivity (Thorburn 2015).
Most swiftlets are highly colonial organisms in large part due to their high degree of habitat specificity. Desirable caves or waterfalls may lead to breeding congregations of thousands of birds with some locations housing multiple species of swiftlet (Medway 1962). These enormous caves, particularly those on the island of Borneo, can contain well over a million individual birds (Sabah Forest Department 1987). The depth at which swiftlets nest within a cave is dependent on the individual species sensory capabilities. Those species capable of full echolocation (see perception section) generally nest far deeper into the cave where light does not penetrate (Medway 1962). Like all members of Apodidae swiftlets are highly aerial birds which avoid landing in any situation. They routinely spend the entire day on the wing prior to returning to their nests at night. This is accommodated by their diet composed of nearly exclusively aerial insects (Lourie and Tompkins 2000).
Swiftlets are most remarkable for the use of echolocation in some species for navigation in low light environments. By emitting a "rattle-like" call composed of a series of short clicks these birds can orient themselves in complete darkness and when blindfolded (Medway 1967). Not all members of Tribe Collacaliini have this ability. It was traditionally thought that only members of the genus g. Aerodramus could use echolocation, however, pygmy swiftlets of the genus g. Collocalia have been documented emitting echolocation associated sounds (Price et al. 2004). Swiftlets and oilbirds (Steatornis caripensis) are the only known birds capable of echolocation (Brinklov et al. 2013). These taxa both inhabit dark caves and other low light environments, making this an exceptionally useful adaptation. In addition to the “rattle-call” swiftlets have social calls used to communicate with members of their own species. These vary from species to species and have been used as a character in the construction of phylogenies (Thomassen and Povel 2006), although social vocalizations are less well studied relative to echolocation associated calls.
All members of Tribe Collocaliini are obligate aerial insectivores. They do not tend to specialize in members of any specific taxon and diets may include a wide range of different species. The most common food items observed are dipterans, homopterans, and hymenopterans (Tarburton 1993). The precise frequency of these items is variable between the different species of swiftlet, especially in areas where they overlap in range (Lourie and Tomkins 2000). Chicks are fed via food ‘boluses’ (a mixture of food matter and saliva) formed by the parents from the same diet (Tarburton 1986). It should be noted that swiftlet echolocation is not sufficiently sensitive to enable detection and pursuit of prey as in bats. Swiftlets are visual hunters and are active during the day, thus this adaptation’s primary function is in navigation.
Swiftlets face predation from a wide range of predators at all life stages. As adults, the birds are hunted by hawk owls while leaving their caves at dawn and dusk and by accipiter hawks throughout the day. At their nesting sites, rats and a broad range of snake species will access their nests. Remarkably, there is a single report of a glossy swiftlet being caught in a spider’s web, with the spider consuming the bird over the course of three days (Manchi and Sankaran 2009). Chicks are significantly more vulnerable to predation. A broad range of species have been documented preying on eggs, hatchlings in the nest, and chicks that have fallen from the nest. These include various snakes, crabs, ants, cockroaches, and lizards; many of which live in colonies beneath swiftlet nesting colonies (Manchi and Sankaran 2009).
Cave ecosystems support extremely diverse communities in association with swiftlet nesting sites (Chapman 1983). This is due to the high energy input introduced to subterranean systems via guano (fecal matter) deposition. A number of organisms are entirely dependent on these deposits and exist only in caves inhabited by bats and swiftlets. These species include fungi, diverse invertebrates, and extensive microbiological assemblages (Gnaspini and Trajano 2000). Most species of swiftlet are host to various varieties of feather lice (Tomkins and Clayton 1999). These parasites have evolved alongside swiftlets and have even been used in the construction of phylogenies (Ledger 1970).
The saliva-based nests of some species of swiftlet in the genus Aerodramus hold great value in certain East Asian markets, particularly China. The nests are made into a bland soup which is purported to alleviate any number of ailments according to traditional medicine. This is of course largely unsupported by the literature (Hobbs 2004). In spite of this, high quality birds nest can cost between $2,000 and $4,000 U.S. dollars per kilogram (Sankaran 2001). Harvesting these nests can be done sustainably (Medway 1966), although there have been concerns in recent years as the volume of nests taken has drastically increased in response to greater Chinese demand for the product (Sankaran 2001).
Of the 27 Collocaliini species assessed by the International Union for the Conservation of Nature , six are at risk of extinction. The Mariana swiftlet is endangered, the Seychelles and Atiu swiftlets are vulnerable, and the waterfall, volcano, and Mascarene swiftlets are near threatened. These species are largely listed due to inhabiting extremely small native ranges that may be easily disturbed by natural disasters, habitat destruction, and invasive species (IUCN). The edible nest swiftlet is currently listed as Least Concern, although there has been some amount of concern regarding overexploitation in the Chinese bird’s nest industry. The species does appear to be declining due to human harvesting at nesting sites, and further monitoring is required to assess the severity of this threat.
Echolocation may have evolved twice in the swiftlets- once in a common ancestor of the Aerodramus swiftlets and then again in pygmy swiftlets. The most recent phylogenies also suggest that it was lost once in giant swiftlets (Price and Johnson 2004). This can likely be explained by the life history of the giant swiftlet. Also known as the waterfall swiftlet, they do not inhabit caves and thus have little need for echolocation. The swiftlet lineage is entirely distinct from that of the South American oilbird, despite similarities in nesting habits and their shared ability to use echolocation for navigation.
James Kelleher (author), Colorado State University, Tanya Dewey (editor), University of Michigan-Ann Arbor.
Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.
living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.
uses sound to communicate
young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.
an animal that mainly eats meat
used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.
The process by which an animal locates itself with respect to other animals and objects by emitting sound waves and sensing the pattern of the reflected sound waves.
parental care is carried out by females
union of egg and spermatozoan
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
An animal that eats mainly insects or spiders.
fertilization takes place within the female's body
parental care is carried out by males
makes seasonal movements between breeding and wintering grounds
Having one mate at a time.
This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.
the area in which the animal is naturally found, the region in which it is endemic.
islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.
found in the oriental region of the world. In other words, India and southeast Asia.
rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.
breeding is confined to a particular season
associates with others of its species; forms social groups.
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
Living on the ground.
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
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