How do birds look at their world? A novel avian visual fixation strategy

Abstract

Abstract: A central assumption in behavioral research is that the observer knows where an animal is looking; however, establishing when an animal is gazing (i.e., visually fixating on an object) has been challenging in species with laterally placed eyes. We quantitatively tested three fixation strategies proposed in the literature for birds, using European starlings (Sturnus vulgaris). We did not find strong support for any of the three strategies, despite high statistical a priori power (93%). However, we did observe a new visual fixation strategy that we labeled monocular alternating fixation. In this strategy, starlings moved their heads to make multiple fixations with a single eye before switching to the other eye and repeating the same process. Additionally, we established that individuals favored using the left over the right eye, supporting that laterality in starlings is left-eye dominant. The newly observed fixation strategy may be associated with the high level of intra-retinal variation (density of photoreceptors, overall sensitivity of visual pigments, etc.) in the starling retina. From a functional perspective, this monocular alternating fixation strategy may be beneficial to integrate the different types of information gathered by the different portions of each retina more quickly. We discuss the implications of our results for designing and interpreting behavioral experiments that require an understanding of where a bird is looking. Significance statement: This is the first study to quantitatively test three hypotheses in the literature about how animals with laterally placed eyes look at objects. We found that there was not strong support for any of these three strategies, but found support for a newly described strategy for birds to look at objects (i.e., multiple looks with a single eye before switching to the other eye).