A Single Drone as Two Observers: Increasing Wildlife Detection Availability in Complex Environments Using Repeated Drone Flights with Offset Paths

Drones are increasingly used to conduct wildlife surveys in complex environments as they can be used in inaccessible areas, quickly survey large areas, and surveys can be reviewed post hoc, often leveraging AI technology. Double-observer methods applied to drone surveys, such as post-survey double-observer review of footage, improve inferences from drone-based wildlife surveys by correcting for individual perceptibility biases of observers. However, occlusion still affects the availability of individuals for detection, especially in complex environments. We explore the possibility that the availability of individuals may be increased by conducting a consecutive repeat survey with a parallel or rotated flight path in a double-observer survey design where individuals are largely stationary. We hypothesized that running this secondary flight would increase the availability of individuals for detection, thereby improving count accuracy, with the most prominent improvement in most complex (e.g., highly vegetated) environments. We explored this using (1) an in-field test of object detectability in environments with differing vegetation, and (2) a simulation of drone wildlife surveys in different levels of occlusion. In both trials, we tested a double-observer detectability estimator tool to correct survey counts. In the field, conducting a secondary flight from a different angle resulted in more accurate counts, with a post-count correction further increasing accuracy. The simulation highlighted detectability benefits from the secondary observer in nearly all trials (99%), and these benefits were generally greater in environments with denser canopies, and from flights paths rotated 90° compared to parallel paths.