Birds are capable of dexterous sensory-motor activities such as tool use. Reaching is a crucial component of tool use and is a vision-guided behavior in primates, in which arm movement is monitored online in a stable visual frame. However, vision-guided reaching in primates is enabled by anatomical separation of the head and arm; neck reaching in birds accompanies head movement, which produces unstable vision because the eye necessarily moves with the bill. This anatomical difference raises the question whether tool use in birds involves visuomotor mechanisms that are distinct from those in primates. As the role of vision in avian tool use has been poorly understood, we investigated the role of vision in tool use in the large-billed crow (Corvus macrorhynchos), a nontool user in the wild. Crows were trained to manipulate an L-shaped hook to retrieve food that was otherwise out of reach. After training, an opaque panel was placed on the front window of the platform to block their vision, and the effects on tool use were tested with respect to performance and movement trajectory. Vision blocking caused similar deviation of tool movement trajectories for both near and far targets, as well as far target-specific deviation. This suggests the involvement of vision in tool use by crows, specifically in the premanipulation process for conversion of vision-body coordinates for motor planning and in the process of tool manipulation. This is the first behavioral evidence for the involvement of vision in avian tool use.
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