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.

What distinguishes many but not all species from other swifts and indeed almost all other birds[1] 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.

Its internal systematics is confusing; the plumage is usually dull, with shades of black, brown, and gray; from their outward appearance, most species are very similar.

Clutch size depends on the location and the food source, but it is generally not large; Aerodramus swiftlets lay 1 to 2 eggs.

Unlike a bat's echolocation, Aerodramus swiftlets make clicking noises that are well within the human range of hearing.

However, it has recently been determined that the echolocation vocalizations do not agree with evolutionary relationship between swiftlet species as suggested by DNA sequence comparison.

[10] This suggests that as in bats, echolocation sounds, once present, adapt rapidly and independently to the particular species' acoustic environment.

Instead of twigs, feathers and straw, these swiftlets make their nest only from strands of their gummy saliva, which hardens when exposed to air.

Eating swiftlet nest material is believed to help maintain skin tone, balance qi ("life energy") and reinforce the immune system.

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).

Guano from both the swiftlets and the many bats that inhabit the caves supports a huge array of specialized animals that feed on the dung.

The Philippine municipality of El Nido in Palawan, known for its limestone cliffs and pristine beaches, is home to a thriving bird's-nest market.