Rotational Stereo-Videography (RSV): a field method for 3D tracking of flying animals, in large air volumes
Abstract
We present RSV, an optical method for reconstructing flight paths, based on stereo-videography and aiming-angle recording. A single filming device is used, combining a single camera and telephoto lens, a set of mirrors and two rotary encoders. The operator aims at the flying animal, and actively tracks it by rotating the device, keeping the animal within the camera’s field of view. For each video frame, the animal can be positioned, within a spherical volume of interest (VOI) centered on the device. The VOI radius depends on the desirable positional uncertainty (measured as the random error, i.e. the 3D position SD), and in turn constrains the maximal tracking duration. We show that short flight bouts of a few seconds, appropriate for flight kinematics analysis (SD < 0.1 m) can be measured within a radius of about 50 m from the observer. Longer flight paths, up to a few minutes long, allowing spatial behaviour investigation (SD < 1 m), can be recorded within about 200 m. At 500m from the device, RSV approaches GPS-like uncertainty (i.e. SD ≈ 5 - 10 m). We share example tracks recorded at various ranges, including unpublished prolonged flight tracks of common swifts (Apus apus). We discuss the strengths and limitations of RSV compared to other local flight tracking techniques (static multi-camera videography, ornithodolite, etc.). We also point out the potential complementarity of RSV tracking at the local scale, with GPS tracking at the global scale, to better understand spatial behaviour processes, in a movement ecology perspective.