Budgerigars adopt robust, but idiosyncratic flight paths

We have investigated the paths taken by Budgerigars while flying in a tunnel. The preferred flight trajectories of nine Budgerigars (Melopsittacus undulatus) were reconstructed in 3D from high speed stereo videography of their flights in an obstacle-free tunnel. Individual birds displayed highly idiosyncratic flight trajectories that were consistent from flight to flight over the course of several months. We then investigated the robustness of each birds trajectory by interposing a disk-shaped obstacle in its preferred flight path. We found that each bird continued to fly along its preferred trajectory up to a point very close to the obstacle before veering away rapidly, making a minimal deviation to avoid a collision, and subsequently returning to its original path. Thus, Budgerigars show a high propensity to stick to their individual, preferred flight paths even when confronted with a clearly visible obstacle, and do not adopt a substantially different, safer route. Detailed analysis of the last-minute avoidance manoeuvre suggests that a collision is avoided by restricting the magnitude of the optic flow generated by the obstacle to a maximum value of about 700 deg/sec. The robust preference for idiosyncratic flight paths, and the tendency to pass obstacles by flying above them, provide new insights into the strategies that underpin obstacle avoidance in birds. It could also have wide-ranging implications for conservation efforts to mitigate collisions of birds with man-made obstacles - especially obstacles that are poorly visible, such as wind turbines or buildings with glass facades. Our findings indicate that care needs to be exercised to ensure that newly planned structures are not located near major bird flyways, wherever possible, and to ensure that the positioning takes into consideration the cues and behaviours that birds use to avoid such obstacles.