Playback-aided surveys and acoustic monitoring in the detection of the Endangered Forest Owlet Athene blewitti

Abstract

Long-term monitoring of populations of rare, endangered species is often challenging. Both the availability of baseline historical datasets and appropriate methods for long-term monitoring are often limited. Anthropogenic climate change and landscape change can impact species distributions significantly, sometimes resulting in a distributional shift of species and, in some cases, driving species to extinction. The Forest Owlet is an endangered bird that was considered extinct but was rediscovered after 113 years in 1997. Since its rediscovery, followed by the description of its calls, there have been regular sightings of the species from newer locations, leading to its down-listing in the IUCN Red List from Critically Endangered to Endangered. One area of interest is the Dang region in Gujarat, India, where there have been no historical records despite previous ornithological studies, but there are several recent records. Through field surveys, we repurpose data from the last three decades (1990, 2000, and 2019) to explore if this bird currently occurs in previous study sites where it was not recorded. The period has seen the rise of acoustic data, and we assess if new survey techniques using playback of its call could increase its detection. Additionally, we examined any changes in landscape and climate in this region across the same period. We also developed an acoustic detection framework for detecting the Forest Owlet from co-occurring sympatric owlets using Automated Recording Units (ARU) and sound-analysis software. We assessed appropriate detection distances from vocalizing birds on the field to design a spacing grid for future surveys of the species. We could locate the Forest Owlet from the resurveys at locations where previous studies three decades ago had not. We also find a significant change in the landscape - loss of native forests and increased agriculture, along with a significant change in climatic variables - mean maximum temperature and mean rainfall. Although the detection of the Forest Owlet is higher when accompanied by playback of its call, there is considerable variation across the landscape. Our acoustic detector comparison led us to a detection strategy for long-term monitoring - different approaches for songs and calls, and an effective detection distance of 300m in its habitat. Although the species responds to climate and habitat change, our study cannot determine the cause of the increased reports of this endangered species. All possibilities remain; the increase in the recent records of the species could be from variable detection or changes in climate and land use. However, we do find increased detections with newer survey techniques with bioacoustics, and we recommend they be used with care for future baseline studies that are urgently required.