Figure 1: Range map of the Edible-nest Swiftlet, adapted from HBW (2018).
8 current subspecies are recognized as shown below (with range and comparative descriptions):
- A. f. amechanus (Oberholser, 1912) – Anambas Is, off SE Peninsular Malaysia.
- Paler underparts with greyer rump than germani
- A. f. germani (Oustalet, 1876) – Coastline from W Hainan S around SE Asia to Malay Peninsula, including Mergui Archipelago (off S Myanmar); coastal N Borneo and W Philippines (Palawan E to Panay and Ticao).
- Paler underparts and whitish rump
- A. f. inexpectatus (A. O. Hume, 1873) – Andaman Is and Nicobar Is.
- Slightly smaller than nominate race
- A. f. vestitus (Lesson, 1843) – Sumatra, Belitung I and Borneo (except N coasts).
- Darkest upperparts, lack contrasting rump
- A. f. perplexus (Riley, 1927) – Maratua I, off E Borneo.
- Some purple sheen on rectrices and remiges, slight contrasting rump
- A. f. fuciphagus (Thunberg, 1812) – Java, Kangean Is and Bali to W Lesser Sundas (E to Sumbawa), and Tanahjampea.
- A. f. dammermani (Rensch, 1931) – Flores (EC Lesser Sundas).
- Slightly paler rump
- A. f. micans (Stresemann, 1914) – Sumba, Sawu and Timor (C Lesser Sundas).
- Slightly greyer overall with contrasting rump
Several authors believe that this species should be split into two. In that treatment, ssp amechanus and germani are named Germain’s Swiftlet, while the rest of the subspecies retain the name of Edible-nest Swiftlet. However, taxonomical evidence for splitting the species has been unconvincing to many, and this species page will treat the group as a single species under the Biological Species Concept (see Taxonomy section).
Differences amongst subspecies are often subtle and difficult to distinguish in the field due to variations in lighting as well as difficulty in observing constantly fast-moving subjects.
According to the subspecies range, Edible-nest Swiftlets observed in Singapore should be A. f. germani, but specimens collected appeared identical to the nominate race, likely because colonies in Malaysia and Singapore are of the house farmed variety (see House Farming below), which is suspected to be of Javan origin (ssp. fuciphagus). To complicate matters, their feeding ranges are not known and may overlap.
3. Ecology and behaviour
Edible-nest Swiftlets are aerial insectivores that catch arthropods on the wing. Diet analyses have been conducted by examining regurgitated food boluses; hymenoptera, diptera and ephemeroptera made up majority of the food items, while arachnida, coleoptera and hemiptera made up most of the rest.
Natural populations of the swiftlets nest in caves, such as the Niah and Gomantong caves in Borneo, and on the Andaman and Nicobar Islands in the Bay of Bengal. However, due to the practice of farming swiftlets in nest houses (see below), the Edible-nest swiftlets have adopted nesting in man-made buildings as early as the 1880s. (Lim & Cranbrook 2002).
Breeding has been observed to take place year-round but peak in October and February. (Nigel Langham 1979).
- Nest building: Nests are made of salivary excrement that harden into a cement-like material. Each nesting pair will spend about 25 minutes a day to build the nest, which takes about 45 days to complete. (Kang et al 1991). Nests are re-used for subsequent clutches in the future if not harvested. (Nigel Langham 1979).
- Chick development: Each pair of birds would usually lay two eggs and spend an average of 23 days incubating them. The chicks take about 43 days to develop and fledge, with an approximate 50% survival rate (Nigel Langham 1979).
As Edible-nest Swiftlets nest in places which are completely dark, they have evolved the ability to echolocate using clicking sounds, along with many other species from the genus Aerodramus. This ability, however, is not diagnostic of the genus as the Pygmy Swiftlet (Collocalia troglodytes) has been proven to possess this ability too.
Sound recording of an Edible-nest Swiftlet's echolocating clicking calls. (Recorded by Lim Hong Yao, 2018)
4. Human interactions
4.1 Bird's nest soup
The Edible-nest Swiftlet’s nests are frequently prepared in a manner known colloquially as ‘birds’ nest soup’, which has long been a Chinese delicacy of immense value since the T’ang Dynasty (618–907 A.D.) Lau and Melville (1994). Like many other Chinese delicacies, birds’ nest soup is not especially tasty, and relies on other culinary additions to appeal to the palate (Thorburn 2015). Its value lies in its rumoured nutritional value and health benefits instead; the broth is said to help reduce inflammation, alleviate gastric problems, boost the immune system, metabolism, and mental performance, as well as improve skin complexion (Marcone, 2005). Several studies have shown that certain claims may hold some truth to them, as components such as sialic acid and other compounds that are purported to aid infant development and immunity were discovered to be present in the nests (Oda et al. 1998; Wang and Brand-Miller 2003; Ng et al. 1986; Kong et al. 1987; Roh et al. 2012). The Black-nest Swiftlet (Aerodramus maximus) also produces nests that are being collected and consumed, but their nests are worth less since they insert their feathers into the nests as additional material, which have to be removed to be eaten.
Given the high valuation of the nests (about USD$20 per nest (Marcone, 2005), nest harvesting naturally became a highly profitable exploit. Despite efforts to keep nesting colonies secret and manage the harvest rates, wild populations of the Edible-nest Swiftlets have plummeted from once immeasurable numbers, and many colonies have now been wiped out, or are critically endangered (Lau and Melville 1994; Sankaran 2001). Italy proposed to include the species under Appendix II of CITES to regulate trade in 1994, but opposition from the Southeast Asian nations prevented it (Thorburn 2015).
4.3 House Farming
Two centuries ago, the Edible-nest Swiftlet nested solely on coastal habitats where caves were available, and there were no records of inland nesting colonies (Cranbrook et al. 2013). Swiftlets were first reported to nest in houses near cliffs in the village of Sedayu in East Java in 1880, and entrepreneurs eventually began developing methods to attract swiftlets to nest in houses (Lim and Cranbrook 2002). The industry grew as the use of broadcasting swiftlet calls to attract nesting birds became the norm, and a trade in swiftlet eggs began. House nesting swiftlets first appeared in the Malay Peninsula in the 1930s, reportedly first arriving in Singapore from Java, before spreading North across Malaysia (Chasen 1939; Gibson-Hill1948). The swiftlets have a high degree of nest fidelity, and house farmed swiftlets and their young would return to man-made houses (preferably the same one) to nest again (Kang et al. 1991), leaving the depleted natural populations and habitats unrestored (Cranbrook et al. 2013). Furthermore, house nesting swiftlets are suspected to be a hybrid population of A. f. fuciphagus and A. f. inexpectatus or A. f. germani in what could be an ongoing case of domestication; the population of house farmed swiftlets have since spread Northward up to Myanmar, where it may be competing with local subspecies. More extensive regional genetic studies are required to confirm these suspicions.
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5. Taxonomy and phylogenetics
5.1 Species Taxonomy
Thunberg (1812) first named the Edible-nest Swiftlets in Java Hirundo fuciphagus, mistakenly placing it together with the swallows (passerines). By the 1900s swiftlets were recognized as a separate taxon and all its members were lumped in the genus Collocalia, then subsequently divided into 3 genera: Hydrochous (Giant Swiftlets) are sister to Aerodramus (medium sized brownish swiftlets), and Collocalia (small glossy plumaged swiftlets) are the basal group that are more related to the swifts. (Cibois et. al 2018)
Several studies have attempted to resolve the phylogenetic tree for swiftlets, but success has been rather limited for the Edible-nest Swiftlet as it does not show up as a monophyletic clade on the tree; results often show that one or more individuals of A. fuciphagus are more closely related to the sister species A. salangana (Mossy-nest Swiftlet) than other A. fuciphagus (Cibois et. Al, 2018; Price et. Al, 2005; Rheindt et. al, 2014). The most recent tree constructed using mitochondrial and nuclear DNA analysis is shown on the right (Figure 2) (Cibois et. al, 2018). The low genetic divergence between closely related taxa is likely due to occasional hybridisation events, which have been reported in Sabah (Lee, 1996), leading to genetic introgression in the form of mitochondrial DNA sweeps (Rheindt & Edwards, 2011). It is evident that mitochondrial DNA is not very effective as a taxonomic indicator in the case of the Aerodramus swiftlets, and genomic methods may be a better alternative to resolve these relationships.
Figure 2: Phylogenetic tree of Aerodramus swiftlets obtained using nuclear and mitochondrial DNA analysis. Adapted from (Cibois et. al, 2018)