How do birds look at their world? A novel avian visual fixation strategy
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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.
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).
KeywordsVision Laterality European starling Fixation Avian vision
We would like to thank three anonymous reviewers or their helpful comments. Hannah Smith (HS) was very helpful in collecting the data from the videos. We are grateful to the USDA APHIS in Ohio who provided the starlings for this experiment, as well as Tom Gnoske from the Field Museum of Natural History in Chicago, IL, who graciously prepared and donated one of our Cooper’s hawk models, and Dr. Barney Dunning from the Department of Forestry and Natural Resources at Purdue University for allowing us to borrow another Cooper’s hawk model.
This research was funded by Animal Behavior Society grant to SRB, a Purdue Research Foundation grant, and a National Science Foundation Grant IOS 1146986. The experiment in the supplementary material was financially supported by the Knox College Richter Fund and National Science Foundation Grant IBN 0109024.
Compliance with ethical standards
The Institutional Animal Care and Use Committee of Purdue University (protocol 1306000876) approved all animal-handling and care procedures. The state of Ohio does not require a permit to capture European starlings because starlings are an invasive species. There is also no required paperwork to transport European starlings from Ohio to Indiana according to the State and Federal organizations (Ohio Department of Natural Resources: Wildlife; Indiana Department of Natural Resources: Fish and Wildlife; Unites States Department of Agriculture, APHIS, Wildlife Services). For the experiment in the supplementary material, procedures were approved by the Institutional Animal Care and Use Committee of Franklin & Marshall College (Protocol # 2000-05).
Conflict of interest
The authors declare that they have no conflict of interest.
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