House Swift
Apus nipalensis
The house swift looks quite similar to the little swift. It is a small bird, but a medium-sized swift, with a wingspan of 12.5-14.5 cm. Its body size ranges from 14 to 16 cm and weighs approximately 20-35 g. This species has a dark, slightly forked tail. Other than the white rump band and white throat-patch, the house swift has a black plumage, darker than the little swift. Voice: House swifts make a ti-ti-ti-tititrrrrrr sound, among many others, as part of their vocal catalogues.
ID text from Wikipedia, CC BY-SA 4.0
Little Swift
Apus affinis
Little swifts are readily identified by their small size. Their wingspan is only 33 cm compared to 42 cm in the case of common swift. Their plumage is black except for a white throat and rump, the white rump patch extending onto the flanks. They have a short square tail, with all rectrices round-tipped. The flight is fluttering like that of a house martin, and the call is a high twittering. Like other swifts they are very short-legged. The legs are used for clinging to vertical surfaces only.
ID text from Wikipedia, CC BY-SA 4.0
Where you might meet both
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