all objects to which birds are not adapted, and avian visual systems limit birds’ ability to detect these obstacles, rendering them vulnerable to collisions. Birds’ visual anatomy and in-flight behaviors together determine the extent to which they can detect physical obstacles and perceive potential threats mid-flight, particularly when passing through relatively open airspace in longer bouts of flight. Although considerable interspecific differences exist in the perceptual capacity of the eyes—with birds of prey notably tending to have the greatest visual acuity ( Mitkus et al., 2018 )—certain commonalities in skull morphology and direction of gaze exist across groups that limit birds’ ability to detect objects in their flight paths. Most birds have eyes oriented laterally on their heads with non-parallel optical systems due to the degree of physical separation between their eyes. As a result, the binocular fields of many birds are narrow ( Martin, 2007 ). Their limited frontal detection is further encumbered because birds must use their peripheral vision to look in the forward direction, which tends to be less accurate than vision from the center of the eye ( Martin, 2007 ). In addition, the narrowness and limited vertical extent of the binocular field create considerable blind spots in the posterior, dorsal, and ventral directions of most birds ( Martin, 2014 ). Many birds frequently look downward or turn their heads to the side while in longer bouts of flight and in open airspace ( Martin & Shaw, 2010 ), and these head movements account for the majority of birds’ linear gaze ( Gioanni, 1988 ; Eckmeier et al., 2008 ). Therefore, even subtle adjustments to the position of the head while in flight may render birds relatively blind to the direction of travel. Historically, frontal detection in open airspace has likely been relatively less important to high-flying or migrating birds due to the lack of tall physical obstructions in natural landscapes. Because of this

Strategies for Pre-Emptive Mid-Air Collision Avoidance in Budgerigars | PLOS One Browse Subject Areas ? Click through the PLOS taxonomy to find articles in your field. For more information about PLOS Subject Areas, click here. ## Figures ## Abstract We have investigated how birds avoid mid-air collisions during head-on encounters. Trajectories of birds flying towards each other in a tunnel were recorded using high speed video cameras. Analysis and modelling of the data suggest two simple strategies for collision avoidance: (a) each bird veers to its right and (b) each bird changes its altitude relative to the other bird according to a preset preference. Both strategies suggest simple rules by which collisions can be avoided in head-on encounters by two agents, be they animals or machines. The findings are potentially applicable to the design of guidance algorithms for automated collision avoidance on aircraft. Citation: Schiffner I, Perez T, Srinivasan MV (2016) Strategies for Pre-Emptive Mid-Air Collision Avoidance in Budgerigars. PLoS ONE 11(9): e0162435. https://doi.org/10.1371/journal.pone.0162435 Editor: Daniel Osorio, University of Sussex, UNITED KINGDOM Received: A