The general shape and blackish plumage recall its relative, the common swift, from which it is distinguished by a white rump band and heavily marked underparts.

The nest is a half-cup of dry grass and other fine material that is gathered in flight, cemented with saliva and attached to a vertical surface.

When the parents cannot find sufficient food in bad weather, the young can survive for days without being fed by metabolising body fat.

The Pacific swift has a large population and extensive breeding area, and faces few threats from predators or human activities.

The Pacific swift is in the Old World genus Apus, which is characterised by dark, glossy plumage, a forked tail and sharply pointed wings.

[1][6] A 2012 paper showed that cooki is closely related to the dark-rumped swift, A. acuticauda, which should therefore be included in the pacificus clade, but made no further taxonomic recommendations.

The southern subspecies, A. p. kurodae, has a narrower white rump (15 mm/0.6 in against the nominate form's 20 mm/0.8 in), a grey throat and blacker underparts.

[3] Juveniles of migratory Apus swifts have a partial moult prior to migration, but retain the larger wing feathers.

The Pacific swift can be distinguished with care by its deeper tail fork, longer wings, bigger head, larger white throat patch and patterned underparts.

[16] In parts of Southeast Asia, migrating Pacific swifts pass through the resident ranges of former subspecies, and good views are then necessary to be sure of correct identification.

[3] The calls given by flocks near the breeding areas are typical swift screams, including a trilled tsiririri or harsher spee-eer.

[17] The nominate subspecies, A. p. pacificus, breeds in eastern Asia from the Ob River northeast to Kamchatka and east to the Kuril Islands, Sakhalin and Japan.

[3][5] Subspecies A. p. kurodae breeds from southeastern Tibet through eastern China to southern Japan, Taiwan and Orchid Island.

In the US, this species is casual in the Pribilof and Aleutian Islands;[3] a claimed 2010 sighting from the Yukon will be the first for Canada and the mainland of North America if ratified.

[21] A mainly aerial species, this swift is not limited to particular land habitats or climatic zones; it breeds from the Arctic to sub-tropical China, and from sea level to at least 3,000 m (9,800 ft) in Japan.

[22] Most Apus swift species nest in rocky areas, and the majority will accept human habitations as a substitute for natural sites.

The Pacific swift is a colonial species that nests 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.

Young birds reach a maximum weight heavier than their parents; they can cope with not being fed for long periods of time, and delay their feather growth when undernourished.

During bad weather, increased competition leads to malnourishment within populations, where young swifts are often not fed for days and survive on stored body fat.

Some birds may die through misadventure or become exhausted when lost on migration (the first record for the Western Palaearctic was found resting on a North Sea gas platform),[36] but swifts have high survival rates and are generally long-lived.