The Pacific swift is a highly aerial bird that breeds in eastern Asia. It is similar in general shape to the Common swift, although slightly longer-winged and with a more protruding head. The fork of the tail is deeper, and the rump is broader. The upper parts are black, apart from a white rump band and a somewhat greyer head. The underparts are black, although white fringes to the feathers give the belly a scaly appearance when seen well from below. The tail and the upper wings are black, and the underwings are brown. The eyes are brown and the small bill and very short legs are black. The sexes are identically plumaged, and juveniles differ from the adults only in that the feathers show pale fringes, particularly on the wings.
Pacific swifts breed in eastern Asia and spend the northern hemisphere's winter in Southeast Asia and Australia. They can be found in a wide range of climatic zones and habitats. Pacific swifts typically breed in sheltered locations such as caves, natural rock crevices, or under the roofs of houses. They tend to winter in lowlands, and in Australia, they are found in arid areas as well as in towns and on the coast.
Pacific swifts are gregarious and even nest in colonies. In Siberia, they usually feed at dusk, sometimes until midnight, and migrants have been seen flying with bats in the Philippines. Pacific swifts tend to hunt higher than most of their relatives, typically at heights up to 300 m (980 ft), only flying close to the ground in poor weather. They often forage near low-pressure areas, which serve both to raise insects from the ground and to give the swifts additional lift. The swifts circle through the insect swarms in flocks typically of tens or hundreds of birds, although sometimes reaching tens of thousands in Australia. Due to increased competition during bad weather young swifts are often not fed for days and survive on stored body fat. The main call of Pacific swifts is a screech usually given by flocks near the breeding areas, including a trilled 'tsiririri' or harsher 'spee-eer'. Pacific swifts are less vocal on the wintering grounds but produce a variety of twitters and buzzes.
Pacific swifts are monogamous and form pairs. Colonies typically nest in sheltered locations such as caves, crevices in vertical rock faces (including sea-cliffs), or under the eaves of houses. The nest is a half-cup of feathers, dry grass, and other light vegetation collected in flight, cemented with saliva, and attached to a ledge or vertical surface with the same substance. The female lays 2 or 3 eggs that are white, as with all swifts. They are incubated by both parents for about 17 days prior to hatching as unfeathered and blind altricial chicks. Both adults brood and feed the chicks, which fledge in an average of 40 days.
There appear to be no significant threats to the Pacific swift. Some birds may die through misadventure or become exhausted when lost on migration, but swifts have high survival rates and are generally long-lived.
According to IUCN, the Pacific swift is locally common throughout its range but no overall population estimate is available. National population estimates include less than 10,000 breeding pairs and around 1,000-10,000 individuals on migration in China; around 100-10,000 breeding pairs and around 50-1,000 individuals on migration in Taiwan; around 100-100,000 breeding pairs and around 50-10,000 individuals on migration in Korea; around 10,000-100,000 breeding pairs and around 1,000-10,000 individuals on migration in Japan and around 10,000-100,000 breeding pairs and around 1,000-10,000 individuals on migration in Russia. Currently, this species is classified as Least Concern (LC) on the IUCN Red List and its numbers today are stable.