Thermal drone observations capture fine-scale population decline of short-tailed shearwaters

Monitoring seabird populations is increasingly urgent as numerous species become more vulnerable to climate change and urbanisation. Surveying burrow-nesting seabirds is challenging due to their nocturnal behaviour, the inaccessibility of colonies, and the disturbance that monitoring poses to nesting sites. Traditional survey methods, which are manual transects conducted by researchers (~200 m), extrapolate this data to derive the population estimates of entire colonies. To enhance the accuracy beyond interpolated data, a survey method was developed using Unoccupied Aerial Systems (UAS) equipped with thermal sensors to survey short-tailed shearwaters (Ardenna tenuirostris). Thermal imagery of breeding colonies was collected from 2019 to 2024, providing comprehensive coverage capturing all occupied burrows (chick presence) at each colony. Occupied burrow densities decreased from 0.28 to 0.18 burrows per m² over this period. Chick numbers decreased by 27% from 2019 (6129) to 2024 (4445). Burrow occupancy counts varied widely (0%–66%) with transect location, highlighting the advantages of using UAS-mounted thermal sensors for providing spatially complete data. This indicates that counts are not uniform, highlighting the bias of using transect data to estimate chick production. A series of simulated transects were imposed over the thermal imagery to compare whole colony chick counts with extrapolated counts. Using data from this study, we estimated that the global breeding population of short-tailed shearwaters is currently 13.5 million, which is approximately 41% less than the last reported global estimate in 1985 of 23 million. This study highlights the utility of emerging technology that addresses the challenges of studying species that are nocturnally active or in remote/inaccessible habitats.<p></p>