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A novel system for bi‐ocular eye‐tracking in vertebrates with laterally placed eyes

Animals use vision to gather information about their environment and then use that information to make behavioural decisions that affect fitness. They will often move their heads or eyes to inspect areas of interest with their centres of acute vision, such as foveae, to gather high resolution information about potential mates, predation risks, or other aspects of the environment. Few studies to date have been able to accurately determine where laterally eyed animals direct their visual attention and how they use their eyes to gather information. We present a non‐invasive eye‐tracking system that can simultaneously track the gaze of two eyes. This is particularly useful for studying animals with laterally placed eyes (most vertebrates) where the two eyes are viewing different images. This system can also accommodate comparative studies using animals of varying size, including small animals that are not frequently used in eye‐tracking studies due to constraints of existing eye‐tracking systems. We conducted an eye‐tracking experiment with European starlings (Sturnus vulgaris) to test the eye‐tracking system, calibration methods and highlight relevant aspects of experimental design. We were able to accurately track the gaze of European starlings with <5 degrees of error. We also found that starlings are more likely to fixate on biologically relevant visual stimuli (e.g. predators and active prey) than simple stimuli (e.g. a dot) in video playbacks. The method presented here can be used to address ecological and evolutionary questions about where animals direct their attention and how they visually inspect mates, food and predators, as well as address management questions about how animals inspect man‐made objects. This method can also be used to answer fundamental questions about vision, such as how laterally eyed vertebrates coordinate the use of their eyes laterally and binocularly.