A seabird's eye view: visual fields of some seabirds (Laridae and Procellariidae) from tropical latitudes.

The visual field of a bird defines the amount of information that can be extracted from the environment around it, using the eyes. Previous visual field research has left large phylogenetic gaps, where tropical bird species have been comparatively understudied. Using the ophthalmoscopic technique, we measured the visual fields of seven tropical seabird species, to understand what are the primary determinants of their visual fields. The visual field topographies of the seven seabird species were relatively similar, despite the two groups of Terns (Laridae) and Shearwaters (Procellariidae) being phylogenetically distant. We propose this similarity is due to their largely similar foraging ecology. These findings support previous research that foraging ecology rather than relatedness is the key determining factor behind a bird's visual field topography. Some bird species were identified to have more limited binocular fields, such as Brown Noddies (Anous stolidus) where binocularity onsets lower down within the visual field, resulting in a larger blind area about the head.