Pacific Swift

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-to-54-cm (17-to-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 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. 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. This species is usually straightforward to identify. …

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. It is strongly migratory, wintering in southern Indonesia, Melanesia and Australia, including Tasmania. It is a common migrant through coastal Malaysia, Sumatra and Java with "vast numbers" crossing the Strait of Malacca. Subspecies A. p. kurodae breeds from southeastern Tibet through eastern China to southern Japan, Taiwan and Orchid Island. It is a relatively short-distance migrant, wintering in the Philippines, Malaysia and northern Indonesia. As a powerful long-distance migrant, the nominate subspecies of Pacific swift has occurred as a vagrant far from its normal range. Birds have been recorded from Brunei, the Maldives, New Zealand and Macquarie Island, and there have been multiple occurrences in the Seychelles. In the US, this species is casual in the Pribilof and Aleutian Islands; a claimed 2010 sighting from the Yukon will be the first for Canada and the mainland of North America if ratified. In South America, there is a 1959 record from Colombia. There are 13 European records as of 2013, from Denmark (two), Spain, Sweden (four) and the UK (seven). It is possible that this overstates the true number of visiting birds. …

All swifts feed on insects caught in flight, and the Pacific swift has been recorded as consuming bees, wasps, termites, moths and flies. A Chinese study found that it caught a wide variety of insect prey and considered that most of the species eaten were harmful to agriculture or forestry, leading to improved agricultural yield in a number of geographical domaines. The Pacific swift tends to hunt higher than sympatric swifts, sharing its airspace mainly with white-throated needletails. It typically feeds at heights up to 300 m (980 ft), only flying close to the ground in poor weather. It often forages 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. In Siberia, Pacific swifts feed at dusk to much later hours than the common swift, sometimes until midnight, and migrants have been seen flying with bats in the Philippines. The young are fed balls of insects bound with saliva. 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.