 At 17 – 18 cm (6.7 – 7.1 in) in length, the Pacific swift is the largest of the Apus swifts. It has a 43 – 54-cm (17 – 21-in) wingspan. Females are slightly heavier than males, averaging 44.5 g (1.57 oz) against 42.5 g (1.50 oz). 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 upperparts are black, apart from a white rump band and a somewhat greyer head. The underparts are black, although white fringes to the feathers gives 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. 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. Juveniles of migratory Apus swifts have a partial moult prior to migration, but retain the larger wing feathers. The moult is completed in the wintering grounds, where adults have a complete moult.
