The Common Swift (Apus apus) is a medium-sized bird, superficially similar to the Barn Swallow or House Martin but somewhat larger. It is, however, completely unrelated to those passerine species, since swifts are in the separate order Apodiformes. The resemblances between the groups are due to convergent evolution reflecting similar life styles. Swifts nearest relatives are thought to be the New World hummingbirds and the Southeast Asian treeswifts.
Their scientific name comes from the Ancient Greek words α "without", and πούς, "feet". ἄπους, apous, meaning "without feet". These birds have very short legs which they use only for clinging to vertical surfaces (hence the German name Mauersegler, literally meaning "wall-glider"). They never settle voluntarily on the ground, where they would be vulnerable to accidents and predation.
The Common Swift was one of the many species described by Linnaeus in his Systema naturae in 1758. The predecessor of the Central European subspecies which lived during the last ice age has been described as Apus apus palapus.
Common Swifts are 16–17 cm long with a wingspan of 38–40 cm and entirely blackish-brown except for a small white or pale grey patch on their chins which is not visible from a distance. They have a short forked tail and very long swept-back wings that resemble a crescent or a boomerang.
The call is a loud scream in two different tone pitches, of which the higher one is from the female and the lower one from the male. They often form 'screaming parties' during summer evenings where about 10-20 Swifts will gather and fly around their nesting area, all calling out to each other, and being answered by other Swifts within the nests. Larger "screaming parties" are formed at higher altitudes, especially late in the breeding season. The purpose of these is not known, but it appears to be the case that these parties, or many Swifts in them, will then ascend to sleep on the wing, while still breeding adults tend to spend the night in the nest.
Like Swallows, Common Swifts are migratory. Their summer breeding range runs from Spain and Ireland in the West across to China and Siberia in the East. They breed as far South as North Africa (in Morocco and Algeria) with a presence in the Middle East in Israel, Lebanon and Syria, across Turkey including the whole of Europe, as far North as Norway, Finland and most of sub-Arctic Russia. They migrate down to Africa by a variety of routes, so far poorly understood, ending up in Equatorial and Sub-Equatorial Africa, excluding the Cape. The map on this page is old and probably reflects the limits of Linnaeus' knowledge at the time; in fact Common Swifts do not breed in the Indian Subcontinent.
Swifts will occasionally live in old woodpecker nest holes in ancient forests, (some 600 having been reported as nesting in such circumstances in the Bialowieska forest in North Eastern Poland while a small colony lives in woodpecker holes and tree nestboxes at the RSPB's reserve in the Ancient Caledonian Forest reserve at Abernethy in Scotland) and it can be surmised that together with cliffs this was their main nest resource, but with the almost complete removal of ancient forest from their nesting range they have adapted to occupy man-made sites and will build their nests of air-borne material caught in flight and bonded with their saliva, in suitable hollows in buildings, under tiles, in gaps beneath window sills, but most typically in the eaves and in gables. Swifts form reasonably faithful pairs that may last for years, and they will return to the same nesting site and partner year after year, rebuilding their nest when necessary. Swift nests degrade naturally over the long period (40 weeks or so) that the birds are absent on migration. Insect species such as clothes moths, carpet and larder beetles will eat them away, sometimes leaving only the most indigestible elements, typically feather shafts, behind.
Young Swifts in the nest can drop their body temperature and other metabolic elements and become torpid if bad weather prevents their parents from catching insects nearby. In this state they may survive for a few days without food.
Except when breeding, Swifts spend their lives in the air, living on the insects they catch in flight. They drink, feed, and often mate and sleep on the wing.