Guam, Mariana swiftlet counts, 2019-2023

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 impacted 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 at Mahlac, Maemong and Fachi Caves from April 2019 to April 2023. This study demonstrates the potential of thermal and near-infrared videography for improved monitoring of yǻyaguak colonies and nesting activity.