Swiftlet
Swiftlets are a tribe (Collocaliini) of small, insectivorous birds in the swift family Apodidae, distinguished by their specialized adaptations for aerial foraging, colonial nesting in caves or cliffs, and construction of nests primarily from salivary secretions.[1] These birds inhabit tropical and subtropical regions across Southeast Asia, the Indo-Pacific islands, and parts of the Indian Ocean, where they feed on flying insects captured during continuous flight.[1] Several species, particularly in the genera Aerodramus and Collocalia, produce nests composed almost entirely of hardened saliva, which are harvested commercially for bird's nest soup, a delicacy in Chinese cuisine valued for purported medicinal properties.[2][3] Unique among birds, certain cave-nesting swiftlets employ echolocation—emitting high-frequency clicks—to orient themselves and locate nests in total darkness, facilitating their reliance on dim, subterranean habitats.[4][5] The swiftlet nest industry, driven by high market demand, has spurred sustainable farming in artificial structures mimicking caves, though wild populations face pressures from overharvesting and habitat loss.[3][6]Taxonomy and Phylogeny
Classification and Genera
Swiftlets comprise the tribe Collocaliini (swiftlets) within the family Apodidae (swifts), order Apodiformes.[1] This tribe includes 28 species of small, aerially adapted birds primarily distributed across tropical and subtropical regions of the Indo-Pacific and Africa.[7] The classification reflects morphological similarities such as reduced external nostrils and, in many species, echolocation abilities, distinguishing them from other Apodidae tribes like Apodini (true swifts).[8] The swiftlets are divided among four genera: Aerodramus (20 species, including cave-nesting forms like the edible-nest swiftlet A. fuciphagus), Collocalia (4-6 species of smaller glossy swiftlets, such as C. esculenta), Hydrochous (monotypic, containing the larger giant swiftlet H. gigas), and Schoutedenapus (2 species of African swiftlets, e.g., S. myoptilus).[9][8] Taxonomic boundaries remain debated due to subtle morphological differences and historical lumping under Collocalia, with molecular phylogenies since the early 2000s supporting the current splits based on genetic divergence and vocalizations.[10] For instance, Aerodramus species exhibit greater genetic diversity linked to island isolation, prompting ongoing revisions.[11]The genus Aerodramus dominates the tribe numerically and ecologically, encompassing species that construct nests from solidified saliva, a trait exploited commercially in Southeast Asia. Collocalia species are noted for their metallic plumage sheen and smaller size (wingspan ~10-12 cm). Hydrochous gigas, at up to 17.5 cm long, represents the tribe's upper size limit and builds nests incorporating moss. Schoutedenapus taxa, confined to montane Africa, feature whitish underparts and are adapted to high-altitude foraging. These genera reflect phylogenetic clustering confirmed by cytochrome-b sequencing, though hybridization risks in sympatric zones complicate species delimitation.[8][10]