Comparing unmanned aerial systems with conventional methodology for surveying a wild white-tailed deer population

Context. Ungulate populations are subject to fluctuations caused by extrinsic factors and require efficient and frequent surveying to monitor population sizes and demographics. Unmanned aerial systems (UAS) have become increasingly popular for ungulate research; however, little is understood about how this novel technology compares with conventional methodologies for surveying wild populations. Aims. We examined the feasibility of using a fixed-wing UAS equipped with a thermal infrared sensor for estimating the population density of wild white-tailed deer (Odocoileus virginianus) at the Cedar Creek Ecosystem Science Reserve (CCESR), Minnesota, USA. We compared UAS density estimates with those derived from faecal pellet-group counts. Methods. We conducted UAS thermal survey flights from March to April of 2018 and January to March of 2019. Faecal pellet-group counts were conducted from April to May in 2018 and 2019. We modelled deer counts and detection probabilities and used these results to calculate point estimates and bootstrapped prediction intervals for deer density from UAS and pellet-group count data. We compared results of each survey approach to evaluate the relative efficacy of these two methodologies. Key results. Our best-fitting model of certain deer detections derived from our UAS-collected thermal imagery produced deer density estimates ((X) over bar = 9.40, 95% prediction interval = 4.32-17.84 deer km(-2)) that overlapped with the pellet-group count model when using our mean pellet deposition rate assumption ((X) over bar = 7.01, 95% prediction interval = 4.14-11.29 deer km(-2)). Estimates from our top UAS model using both certain and potential deer detections resulted in a mean density of 13.77 deer km(-2) (95% prediction interval = 6.64-24.35 deer km(-2)), which was similar to our pellet-group count model that used a lower rate of pellet deposition ((X) over bar = 10.95, 95% prediction interval = 6.46-17.65 deer km(-2)). The mean point estimates from our top UAS model predicted a range of 136.68-273.81 deer, and abundance point estimates using our pellet-group data ranged from 112.79 to 239.67 deer throughout the CCESR. Conclusions. Overall, UAS yielded results similar to pellet-group counts for estimating population densities of wild ungulates; however, UAS surveys were more efficient and could be conducted at multiple times throughout the winter.