Phylogenetic and ecological significance of the variation in the scleral ring of aquatic forging birds
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Diving birds must accommodate their vision to air and water, two very different optical media. A few species, such as penguins (Sphenisciformes), dive to depths approaching the aphotic zone of the ocean and must accommodate their vision to both the change in media and significant light attenuation. Previous studies show that optical properties of the eye e.g., pupil aperture and focal length, are reflected in the eye’s gross morphology and have a strong positive correlation with osteological proxies from the orbit and scleral ring. This study combines qualitative and quantitative measurements utilized in previous studies to create a larger feature space for classification. A total of 170 species were evaluated for distinct classes of eye shape, 87 of which were newly measured species of Charadriiformes, Procellariiformes, "Pelecaniformes", and Sphenisciformes. Unlike in previous studies of land birds, no pattern was recovered in eye shape based on the light levels where the birds foraged. Aquatic birds are active in a wide range of conditions; foraging both day and night and at varying water depths based on the time of year and available prey. Diverse aquatic bird ecology probably contributes to the lack of distinct eye shape classes based on dive depth. Regression analysis was able to recover prey capture style (i.e., plunge, pursuit, skimming, or wading) based on orbit and scleral ring measurements. Misclassification was most common in taxa that exhibited multiple modes of prey capture such as the albatross (Diomedeidae). The ability to determine foraging style based on osteological eye measurements may have implications for identifying prey capture method in extinct species of aquatic birds.