Rules to fly by: pigeons navigating horizontal obstacles limit steering by selecting gaps most aligned to their flight direction

Abstract

Flying animals must successfully contend with obstacles in their natural environments. Inspired by the robust maneuvering abilities of flying animals, unmanned aerial systems are being developed and tested to improve flight control through cluttered environments. We previously examined steering strategies that pigeons adopt to fly through an array of vertical obstacles. Modeling vertical obstacle flight guidance revealed that pigeons steer toward larger visual gaps when making fast steering decisions. In the present experiments, we recorded 3D flight kinematics of pigeons as they flew through randomized arrays of horizontal obstacles. We found that pigeons still decelerated upon approach but flew faster through a denser array of horizontal obstacles compared with the vertical obstacle array previously tested. Pigeons exhibited limited steering and chose gaps between obstacles most aligned to their immediate flight direction, in contrast to vertical obstacle navigation that favored widest gap steering. In addition, pigeons navigated past the horizontal obstacles with more variable and decreased wing stroke span and adjusted their wing stroke plane to reduce contact with the obstacles. Variability in wing extension, stroke plane and wing stroke path was greater during horizontal obstacle flight. Pigeons also exhibited pronounced head movements when negotiating horizontal obstacles, which potentially serve a visual function. These head-bobbing-like movements were most pronounced in the horizontal (flight direction) and vertical directions, consistent with engaging motion vision mechanisms for obstacle detection. These results show that pigeons exhibit a keen kinesthetic sense of their body and wings in relation to obstacles. Together with aerodynamic flapping flight mechanics that favors vertical maneuvering, pigeons are able to navigate horizontal obstacles using simple rules with remarkable success.