Echolocation click structure, behavioral context and daily rhythm in the House-farm Swiftlets (Aerodramus fuciphagus)

Broadcasting recorded bird calls is a widely used technique in White-nest Swiftlet farming, yet the echolocation of House-farm Swiftlets remains poorly described. In this study, we analyzed 16 24-h audio-video recordings in the main nest area and the swiftlet entrance point of a commercial swiftlet house in Sitiawan, Perak, Malaysia, to characterize three aspects of the swiftlet echolocation: signal structure, behavioral context and diel rhythm. Based on 160 structurally clear double-clicks, we characterized a typical echolocation pattern consisting of 2 closely spaced clicks. Under the present recording and filtering settings, the measured frequency range was predominantly within the audible range, with median lower and upper frequency limits of approximately 1.2 and 8.6 kHz, respectively. The second click was slightly longer than the first and showed a higher maximum frequency and broader bandwidth, indicating a consistent within-pair asymmetry. Using infrared video footage, we assigned echolocation sequences to eight behavioral/movement-related contexts. Pulse repetition rate (PRR) of each category of echolocation sequence showed two distinct levels: (1) lower values (about 6‒8 pulses/s) when entering or exiting the nest, parent-chick interactions and performing parallel shift along the nest boards; and (2) higher values (about 11‒16 pulses/s) during short flights within the nest area, departures, returns and passages through the house entrance. Analysis of 30 min bins across the 16 sample days revealed a consistent bimodal diel pattern in the echolocation activity, peaking at dawn and dusk/early night and a pronounced trough from late morning to afternoon, closely matching previously reported peaks in parental visits and chick provisioning. Our results provide the first quantitative baseline for echolocation use by House-farm Swiftlets and highlight the potential of passive acoustic monitoring to track colony activity and inform acoustics-friendly design and management of these roosts.