Gaze tracking of large-billed crows (Corvus macrorhynchos) in a motion capture system

Abstract

Previous studies often inferred the focus of a bird's attention from its head movements because it provides important clues about their perception and cognition. However, it remains challenging to do so accurately, as the details of how they orient their visual field toward the visual targets remain largely unclear. We thus examined visual field configurations (Study 1) and the visual field use (Study 2) of large-billed crows (Corvus macrorhynchos). Study 1 employed an established ophthalmoscopic reflex technique to identify the visual field configuration, including the binocular width and optic axes, as well as the degree of eye movement. Study 2 used a newly established motion capture system to track the head movements of freely moving crows and examined how they oriented their reconstructed visual fields toward attention-getting objects. When visual targets were moving, the crows frequently used their binocular visual fields, particularly around the projection of the beak-tip. When the visual targets stopped moving, crows frequently used non-binocular visual fields, particularly around the regions where their optic axes were found in Study 1. On such occasions, the crows slightly preferred the right eye. Overall, the visual field use of crows is clearly predictable. Thus, while the untracked eye movements could introduce some level of uncertainty (typically within 15 degrees), we demonstrated the feasibility of inferring a crow's attentional focus by 3D tracking of their heads. Our system represents a promising initial step towards establishing gaze tracking methods for studying corvid behavior and cognition.