When trying to understand what an animal is paying attention to, studying the way it shifts its gaze can reveal a lot about what it's focusing on. But for birds, this can pose a problem as most birds can use both binocular vision – looking down their beak in front of them with both eyes – and non-binocular vision – using one eye at a time looking towards the side. Birds can switch between these two types of vision depending on the type of information they are trying to gather. With this in mind, Akihiro Itahara of Kyoto University, Japan, and Fumihiro Kano of the University of Konstanz, Germany set out to discover where the large-billed crow (Corvus macrorhynchos) was gazing by tracking how they were moving their heads.
But the researchers first needed to find out what the shape of the area the birds could see, or visual field, looked like. To do this, the team shone a light around the crow's head and marked the angle between the eye and the light when the reflection from the crow's retina was half bright and half dark. Using this method, the researchers were able to identify the visual field of each eye, how much overlap the bird had in its binocular field of vision and where the crows had blind spots. If the birds are not using their binocular field of vision, they have a blind spot in front of their head of 25 degrees.
Now knowing what the birds could see using each visual field, the scientists then recorded the movement of the crows’ heads while a colorful object was tossed in the room. Using tiny, lightweight markers attached to the birds’ heads, the scientists were able to track the movement using motion capture software. The duo found that the crows turned their head to use their binocular vision when the object was being shaken by the researcher and continued to move their heads, following the object as it rose and fell to the ground when thrown, suggesting that the birds preferred using their binocular vision to track the moving object. Once the object had stopped moving however, the crows would look at it with only one eye, using their non-binocular visual field to glean different information from the object. While this method allows researchers to understand where crows are focusing their attention, more work will be needed to see if head movements offer glimpses into the gaze of other birds.
