Visual fields and foraging in procellariiform seabirds: sensory aspects of dietary segregation.

Retinal visual fields were determined using an ophthalmoscopic reflex technique in two seabird species of the family Procellariidae: white-chinned petrel Procellaria aequinoctialis and antarctic prion Pachyptila desolata. The binocular fields of both species show a similar shape but they differ in size and in the position of the bill within the field. In white-chinned petrels the binocular field extends vertically through approximately 140 degrees and has a maximum width of approximately 40 degrees. The bill is placed approximately central within the field. The binocular field of the prions is approximately half this width and vertical extent, and the bill is placed close to the ventral edge. These differences in binocular field topography can be correlated with the different foraging techniques that these birds employ when seeking a similar diet within the same environment. White-chinned petrels pursue individual items both at the surface and while diving to moderate depths. Antarctic prions feed primarily by filtering items from surface waters. These differences in visual field topography mirror those found in different terrestrial bird species that primarily employ visual or tactile cues in the pursuit of food items. White-chinned petrel eyes and visual fields show features of an amphibious optical design similar to those found in penguins and albatrosses.