We have investigated the paths taken by Budgerigars while flying in a tunnel. The 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 bird's 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 over the obstacle 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, unobstructed route. 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. We believe that this is the first carefully controlled study of the behaviour of birds in response to a newly introduced obstacle in their flight path. The insights from the study could also have implications for conservation efforts to mitigate collisions of birds with man-made obstacles. Recently, there has been growing interest in understanding how birds cope with the challenges of short-range navigation and guidance. The motivation for these studies has been to understand the principles of visually guided flight in airborne creatures, as well as to enquire whether some of these biologically evolved principles can be applied to the design of algorithms for the guidance of unmanned aerial vehicles. Some aspects of visually-guided flight in birds are now beginning to be investigated-such as flight through narrow tunnels 1,2 , regulation of flight speed 3 , flight between obstacles 4,5 , collision avoidance 6,7 choice of landing locations 8 , flight through narrow apertures 9 , and body awareness 10. However, this challenging area of research is still in its infancy. Much still remains to be learned about the visual cues that are used to guide various manoeuvres, whether the cues used depend upon the context, and whether they are used in the same way across all bird species. In this paper we investigate the paths taken by individual birds when they fly in a 25 m tunnel, which is long enough to encourage flight speeds that approach those in natural outdoor flight. We investigate the following questions: First, do all birds use the same flight trajectory, or does the preferred trajectory vary from bird to bird? Second, if the birds exhibit individually different flight paths, does this idiosyncratic preference persist with the passage of time? Third, if an obstacle is placed in a bird's preferred path, does it switch to an entirely different flight ...
