Pallid Swift
Apus pallidus
This 16–17 cm (6.3–6.7 in) long species is very similar to the common swift, and separation is only possible with good views. Like its relative, 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 a large white throat patch which is frequently visible from a distance. It is chunkier and browner than common swift, and the slightly paler flight feathers, underparts and rump give more contrast than that species. It also has a scalier looking belly and subtly different flight action.
ID text from Wikipedia, CC BY-SA 4.0
White-rumped Swift
Apus caffer
This 14–15.5 cm (5.5–6.1 in) long species has, like some relatives, a short forked tail and long swept-back wings that resemble a crescent or a boomerang. It is entirely dark except for a pale throat patch and a narrow white rump. It is similar to the closely related little swift, but is slimmer, darker and has a more forked tail and a narrower white rump. Voice: This is a quiet species compared to little swift, but a twittering trill is sometimes given.
ID text from Wikipedia, CC BY-SA 4.0
Where you might meet both
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Play BeakBrainPhotos and recordings are credited to their authors under the licence shown. Full profiles: Pallid Swift and White-rumped Swift.