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ESR 53:139-149 (2024)  -  DOI:

Videographic monitoring at caves to estimate population size of the endangered yǻyaguak (Mariana swiftlet) on Guam

P. Marcos Gorresen1,*, Paul Cryan2, Megan Parker3, Frank Alig3, Melia Nafus4, Eben H. Paxton4

1Hawai’i Cooperative Studies Unit, University of Hawai’i at Hilo, Hawai’i National Park, HI 96718, USA
2US Geological Survey, Fort Collins Science Center, Fort Collins, CO 80526, USA
3US Navy, NAVFAC Systems Command Marianas (NAVFACMAR), Guam 96915, USA
4US Geological Survey, Pacific Island Ecosystems Research Center, Hawai’i National Park, HI 96718, USA
*Corresponding author:

ABSTRACT: The yǻyaguak (Mariana swiftlet; Aerodramus bartschi) is an endangered cave-nesting species historically found on Guam and the southern Mariana Islands, Micronesia. The population on Guam has been severely affected by the introduction of the brown treesnake Boiga irregularis. Population status assessments have, however, been challenging due to the limitations of traditional counting methods, which rely on visual observations at cave entrances and are prone to inaccuracies. To improve count accuracy, we estimated yǻyaguak population size and relative nesting activity using thermal and near-infrared videography. The population on Guam was surveyed at the island’s 3 known occupied caves (Mahlac, Maemong, and Fachi) between 2019 and 2023. Mahlac Cave harbored the largest colony, which ranged from 506 to 665 birds; Maemong Cave held 144 to 196 birds; and Fachi Cave, which is sometimes flooded, had 28 (in 2019) and 35 birds (in 2023). Our estimates indicate a slight decline in the yǻyaguak population over the study period. This study demonstrates the potential of thermal and near-infrared videography for improved monitoring of yǻyaguak colonies and nesting activity, which will contribute to our understanding of population dynamics and the effectiveness of management strategies such as brown treesnake control.

KEY WORDS: Swiftlet colony · Autonomous camera systems · Apodidae · Micronesia

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Cite this article as: Gorresen PM, Cryan P, Parker M, Alig F, Nafus M, Paxton EH (2024) Videographic monitoring at caves to estimate population size of the endangered yǻyaguak (Mariana swiftlet) on Guam. Endang Species Res 53:139-149.

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