Detecting spider monkeys from the sky using a high-definition RGB camera: a rapid-assessment survey method?

Abstract

Commercial, off-the-shelf, multirotor drones are increasingly employed to survey wildlife due to their relative ease of use and ability to cover areas quicker than traditional methods. Such drones fitted with high-resolution visual spectrum (RGB) cameras are an appealing tool for wildlife biologists. However, evaluations of the application of drones with RGB cameras for monitoring large-bodied arboreal mammals are largely lacking. We aimed to assess whether Geoffroy’s spider monkeys (Ateles geoffroyi) could be detected in RGB videos collected by drones in tropical forests. We performed 77 pre-programmed grid flights with a DJI Mavic 2 Pro drone at a height of 10 m above the maximum canopy height covering 45% of a 1-hectare polygon per flight. We flew the drone directly over spider monkeys who had just been sighted from the ground, detecting monkeys in 85% of 20 detection test flights. Monkeys were detected in 17% of 18 trial flights over areas of known high relative abundance. We never detected monkeys in 39 trial flights over areas of known low relative abundance. Proportion of spider monkey detections during drone flights was lower than other commonly employed survey methods. Agreement between video-coders was high. Overall, our results suggest that with some changes in our research design, multirotor drones with RGB cameras might be a viable survey method to determine spider monkey presence in closed-canopy forest, although its applicability for rapid assessments of arboreal mammal species′ distributions seems currently unfeasible. We provide recommendations to improve survey design using drones to monitor arboreal mammal populations.