Acoustic Monitoring of the Fruit Bat Colony in Miag-ao, Iloilo
Published July 28, 2026
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Fruit bats flying at night
Every evening at dusk (between 5:30 and 6:30 PM), large colonies of fruit bats emerge from their communal daytime roosts in Miagao, signalling the start of their nightly foraging activity. Individuals disperse across the surrounding landscape, some even believed to reach the far areas of Antique, in search of food and resources before returning to the same communal roosts prior to sunrise, completing a consistent activity cycle of emergence, nocturnal foraging, and roost re-entry.
Fruit bat vocal communication has received increasing scientific attention over the past several decades. Studies of Grey-headed Flying Foxes (Pteropus poliocephalus) have demonstrated that these bats possess multiple distinct call types associated with specific social behaviors, including aggression, courtship, maternal care, alarm, and feeding. Juvenile flying foxes also produce individually distinctive isolation calls that may enable mothers to recognize their offspring. More recent work has shown that Grey-headed Flying Fox vocalizations remain largely unaffected by typical urban noise levels, although extremely loud anthropogenic sounds, such as low-flying aircraft, can temporarily suppress colony vocal activity through what has been termed the “silentium effect.” Advances in machine learning have further demonstrated that several flying fox vocalizations can be automatically recognized and classified in real time.
Research on Egyptian fruit bats (Rousettus aegyptiacus) has expanded understanding of social communication even further. Long-term continuous recordings collected over more than a year have produced one of the world’s largest annotated fruit bat vocalization datasets, documenting hundreds of thousands of social calls together with behavioral observations and vocal development from birth to adulthood. These studies have shown that fruit bat vocalizations contain information about the identity of the caller, the intended recipient, the social context, and the behavioral response, highlighting the complexity of fruit bat communication.
Despite these advances internationally, comparable research has not yet been documented for Philippine fruit bats. Existing bat bioacoustic studies in the Philippines have largely focused on the echolocation calls of insectivorous bats, while long-term investigations of the social vocalizations, and acoustic activity patterns and behavior of Philippine fruit bats remain absent from the published literature. To address this, the Sulu Garden Foundation has started their acoustic monitoring project of the fruit bat population in Miag-ao, a long-term initiative for documenting and analyzing the vocalizations and acoustic activity of the municipality’s fruit bat population. Consequently, this project is poised to become one of the first long-term passive acoustic monitoring programs for fruit bat colonies in the Philippines.
How We Record
The project uses passive acoustic monitoring through a dedicated wildlife sound recorder deployed near the colony’s communal roosting site. The monitored colony consists of more than 105 fruit bats (counted as of June 2026) roosting in a rain tree behind Sulu Garden, Miag-ao, Iloilo. Previous observations have confirmed the presence of the island flying fox (Pteropus hypomelanus), the large flying fox (Pteropus vampyrus), and the little golden-mantled flying fox (Pteropus pumilus). Because these species roost together and their vocalizations have not yet been identified to species level, the recorded calls cannot currently be attributed with certainty to any particular species. Consequently, the acoustic recordings represent the collective vocal activity of the mixed-species colony rather than that of a single fruit bat species.
Continuous audio recordings are collected throughout the week every night to capture the full range of bat acoustic activity, including periods before emergence, during mass departure, throughout nocturnal activity, and prior to the colony’s return at dawn. These recordings are analyzed to identify distinct vocalizations, characterize temporal patterns in acoustic activity, quantify changes in calling intensity, and determine peak periods of colony movement. Long-term datasets will be generated and will enable comparisons across seasons and years, supporting the detection of changes in colony activity, and providing a foundation for future studies on fruit bat behavior, communication, and population monitoring. The resulting archive will then help in the development of better methods for recognizing bat vocalizations and distinguishing biologically relevant calls from environmental sounds.
Equipment used:
– Song Meter Mini 2
– Micro SD
– 8x AA Batteries
Take a Closer Look at the Bats
Aerial view of roosting bats near Sulu Garden taken at 5:30 PM.
Listen to the Colony
Vocalizations while bats remain in the roost before departure (4:21 PM – 4:22 PM)
Peak colony activity during dusk (5:37 PM – 5:38 PM)
Acoustic activity recorded during bats return (4:24 – 4:25 AM)
The project also documents the bats through synchronized photography, videography, and direct field observations. Integrating these will allow researchers to correlate vocal activity with observable behaviors, validate acoustic interpretations, and improve understanding of colony dynamics during different stages of the nightly activity cycle.
Bat roosting site on rain tree behind Sulu Garden
As the project expands, the growing collection of recordings, annotated vocalizations, photographs, and behavioral observations will serve as a valuable scientific resource for researchers, students, conservation practitioners, and the wider community.
Beyond improving our understanding of Philippine fruit bat acoustic ecology, the project is intended to contribute to the long-term study of fruit bat biology and to provide baseline information that supports the conservation and management of this ecologically important, Near Threatened species. The resulting long-term acoustic dataset will provide valuable information for monitoring changes in colony activity, detecting potential shifts in population behavior over time, and informing future conservation and management efforts.
References
Christesen, T., & Nelson, J. (2000). Vocal communication in the Grey-headed Flying-fox (Pteropus poliocephalus).
Nelson, J. (1964). Vocal Communication in Australian Flying Foxes (Pteropodidae; Megachiroptera).
Pearson, T., & Clarke, J. A. (2019). Urban noise and grey-headed flying-fox vocalisations: Evidence of the silentium effect. Urban Ecosystems, 22(2), 271–280. https://doi.org/10.1007/s11252-018-0814-2
Grey-Headed Flying Fox Vocalisation Machine Listening. More-than-Human Futures.
Prat, Y., et al. (2017). An annotated dataset of Egyptian fruit bat vocalizations across varying contexts and during vocal ontogeny. Scientific Data.
Prat, Y., et al. (2016). Everyday bat vocalizations contain information about emitter, addressee, context, and behavior. Scientific Reports.
Van Parijs, S. M., & Corkeron, P. J. (2002). Ontogeny of Vocalisations in Infant Black Flying Foxes, Pteropus alecto. Behaviour, 139(9), 1111–1124. http://www.jstor.org/stable/4535976
About SGF
Sulu Garden Foundation is named after the Sulu Sea – the largest inland sea in the Philippines. Our mission concentrates on this sea and the provinces that surround it. These provinces have diverse cultures and religions, yet, they all share history and traditions. The foundation has various programs and activities focused on history and heritage, the arts, nature conservation, research and development, and community.
