Appearance
The grey-rumped swiftlet is 9–10 cm in length with a square tail. The back and upper surface of the wings are dull dark blue with a moderate green gloss. The rump is paler due to the white margins of feathers forming the uppertail coverts. The throat and upper breast are grey with fine white scalloping merging into larger greyish chevrons over the lower breast and flanks, becoming white over the belly. There is sometimes a tuft of small feathers on the hallux, the rear facing toe. This species lacks the white spots on the inner webs of the tail feathers that are present in some ''Collocalia'' species.References:
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