The endemic subspecies of the Pacific sheath-tailed bat (payeyi; Emballonura semicaudata rotensis), and the Mariana swiftlet (chachaguak; Aerodramus bartschi) once inhabited many of the islands that comprise the Mariana archipelago (Lemke 1986, Flannery 1995, Ellison et al. 2003, Cruz et al. 2008). Both are insectivorous species that roost or nest almost exclusively in natural limestone caves (Pratt et al. 1987). Population threats to these two species primarily include habitat loss from past clearing of native forest for agriculture, with subsequent replacement by invasive vegetation (particularly Lantana camara), habitat degradation from feral goat browsing, persistent disturbance of nests and roosts, pesticide use, predation of swiftlets on Guam by the brown treesnake (Boiga irregularis) and cyclonic storms (USFWS 1991, Esselstyn et al. 2004). Populations of E. s. rotensis are currently extirpated from all of the major islands in the Commonwealth of the Northern Mariana Islands (CNMI), except for a single remnant population on Aguiguan estimated in 2008 to consist of 359 to 466 individuals (Wiles et al. 2011). A 2016 survey of sheath-tailed bats on Aguiguan estimated a minimum of 323 individuals, but that survey was not completed due to heavy rains. Extinct on the islands of Rota and Tinian, populations of A. bartschi in the Marianas currently persist only on Aguiguan, Saipan, and Guam (USFWS 1991, Wiles et al. 2003, Johnson et al. 2018). The swiftlet population on Aguiguan was estimated in 2016 at approximately 338 individuals (Liske-Clark et al. 2018). Both species are listed as endangered by the CNMI government (Anonymous 1991, CNMI 2000), U.S. Fish and Wildlife Service (USFWS 1984, 2015), and International Union for Conservation of Nature (Bonaccorso and Allison 2008).
In general, crepuscular and nocturnally active animals are difficult to detect and monitor. Surveys can be hampered by potential observer bias and the difficulty of distinguishing swiftlets from sheath-tailed bats during low-light conditions. To overcome these problems and assess the population size and cave occupancy of the Pacific sheath-tailed bat and the Mariana swiftlet on Aguiguan, we propose to conduct thermal video-based surveillance of cave entrances. Video-based monitoring systems and motion analysis of digital imagery acquired by thermal infrared cameras have proven effective for quantifying nocturnal activity of bats (e.g., Cryan et al. 2014, Gorresen et al. 2017) and counting swiftlets (Johnson et al. 2018, U.S. Geological Survey, in litt.), without the need to physically contact the animals or enter roost caves.