The plain swift (Apus unicolor) is a medium-sized swift. Although this bird is superficially similar to a barn swallow or house martin, it is not related to those passerine species. The resemblances between the groups are due to convergent evolution reflecting similar life styles.
This 14–15 cm long species is very similar to the closely related common and pallid swifts, which also occur in the archipelagos, and separation is only possible with good views. Like its relatives, it has a short forked tail and very long swept-back wings that resemble a crescent or a boomerang.
It is entirely dark except for an indistinct pale throat patch. It is slimmer and more rakish than the pallid swift, and is darker than that species and lacks the obvious white throat.
Distinguishing the plain swift from the similarly plumaged common swift is much more difficult, although juvenile common can be easily eliminated due to its white throat. The plain is slimmer and appears longer winged than the common, and has scaly underparts, difficult to see except with excellent views. The call is a loud dry scream similar to that of the common swift, though possibly higher pitched.