A small acrobatic swiftlet, often found in large swirling flocks above fields, forests, and settlements. Dark brown above, washed-out gray-brown below; all but identical for the most part to many other swiftlets in range. Rump is marginally paler than Himalayan, Black-nest, and Mossy-nest swiftlets, but not white, as in House Swift. Best told apart by its nest; a pale gelatinous bowl (instead of a dark one) composed of moss or debris, typically located in caves or building overhangs. Gives buzzy trills in flight.