Assessment of the accuracy of counting large ungulate species (red deer Cervus elaphus) with UAV‐mounted thermal infrared cameras during night flights

Abstract

Unmanned aerial vehicles (UAVs) are increasingly used in wildlife surveying, including estimation of population densities. It is essential that we evaluate and test new survey methods to guide optimal sampling strategies. This study aimed to assess the accuracy of using a UAV-mounted thermal infrared (TIR) camera to count red deer Cervus elaphus populations, and how this was influenced by flight season, height and velocity, in order to help guide future census design. We flew 57 flights across a captive population of red deer in a 13 ha deer park enclosure of semi-natural habitat, representative of the species' range in northern Germany. Flights and image assessments were performed with no prior knowledge of actual population size. Accuracy was quantified by comparing real population size (known only to deer park staff) and independently estimated population sizes from UAV TIR images. Accuracy was significantly influenced by ecological season (early and late winter, spring and early summer) and height. Across all seasons, lower flights (100 m) performed better than higher ones (120 m), with lower flights in early winter and early summer being on average accurate to within 1% of actual population counts. For the season where we had the largest range of temperatures between flights (late winter) we found that accuracy was highest when temperatures were lowest. Flights were also able to identify all five stags (defined as a male deer ≥ 2 years old) present in early summer, but not in spring. Deer appeared to avoid the landing/take-off area, but there were no noted behavioural responses to drones flying over animals when at constant height and velocity during surveys. Our results indicate that UAV-mounted TIR camera have the potential to accurately count populations of large ungulate species, but that flight season, height and potentially temperature need to be taken into account to maximise accuracy. This approach has the potential to be scaled up to more accurately estimate densities of wild populations compared to existing approaches.

Camera trap methods and drone thermal surveillance provide reliable, comparable density estimates of large, free-ranging ungulates

“…Simplest, most cost effective, and likely most accurate, however, are simple counts from drone-based thermal videography, which in this study captured population density within a few hours of flights-with subsequent processing and analysis-and has been shown in studies of known captive animals to reproduce known population sizes with high fidelity [31,68,69]. Flights repeated seasonally could capture many of the spatiotemporal patterns revealed by long-term camera trapping and NMM analysis, and the spatial quality of aerial videography enables georeferenced observations of animal movement and short-term patterns of habitat use [32].…”

Use of drone technology in animal health and management

“…This finding suggests that UAVs can be used effectively in wildlife surveys and that this technology can improve management actions. Similarly, a study by Zabel et al [16] evaluated the accuracy of UAVs and thermal infrared cameras in counting red deer populations. The study revealed that factors such as season, flight altitude and temperature affect the accuracy of the UAV.…”

“…The concerns raised by Duporge et al [13] emphasize the ethical use of UAV technology. However, research by Ezat et al [15] and Zabel et al [16] shows that UAVs have potential for accurate population counts and more effective monitoring of wildlife. Therefore, harmonization of regulatory frameworks and ethical guidelines for the use of UAVs in the future may lead to more widespread use of this technology in wildlife research.…”

Dead zones, obstructions, and missed captures: a novel approach for determining the Effective Capture Area for remote camera traps

“…For example, in winter, ungulates have thick winter coats that allow them to retain heat and survive harsh conditions and as such, less heat may be escaping the coat (Parker and Robbins 2018). A lack of heat emission has been previously observed in thermal imaging studies, where thermal cameras have a difficult time picking up certain species during certain temperature ranges (Kuhn & Meyer, 2009;Zabel et al, 2023). As a result, capture probability may be even lower during mid-winter months for some wildlife species.…”