These are the Plume-toed Swiftlet (Collocalia affinis), See: Rheindt, Christidis, Norman, et al (2017). Speciation in Indo-Pacific swiftlets (Aves: Apodidae): integrating molecular and phenotypic data for a new provisional taxonomy of the Collocalia esculenta complex. Zootaxa. 4250 (5): 401–433. They are called “Plume-toed” Swiftlet as there is a tuft of small feathers on the hallux (rear facing toe). I saw a roosting site for these swifts at this hill station. Information from my host suggests the roosting site has been in used for both nesting and roosting for many decades (currently not nesting). There are approximately 300 Plume-toed Swiftlets using a garage to roost every night. They come in around 7pm (dusk) like a swarm of bats and leave at dawn (approximately 6.00 to 6.30am). They will roost by clinging on to old nests, to the bare ceiling (rough stone surface) and even to other birds. While flying in to roost, when initially roosting and when getting ready to leave in the morning they will all call out collectively, in a loud din that is deafening. Waveform and sonogram image provided. Special note: Images were taken with the aid of a torch light (no flash used) and we tried to limit our disturbance. They can be affected by prolonged light disturbance at night.