Visual Fields and Eye Movements in Herons (Ardeidae)

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Abstract
The visual fields and eye movements of three heron species (Ardeidae; Ciconiiformes): the cattle egret (Bubulcus ibis), the squacco heron (Ardeola ralloides), the western reef heron (Egretta gularis schistacea) were determined in alert, restrained birds using an ophthalmoscopic technique. All three species can gain panoramic visual coverage of the frontal field around the bill, and a bird standing with its bill horizontal can view its own feet binocularly. The region in which binocular overlap is possible is long (approximately 170°) and narrow (maximum width approximately 20°). Monocular field width in a horizontal plane is approximately 170°. Retinal binocular overlap can be abolished by eye movements at all elevations in the frontal field. At the frontal margins of the monocular fields the retinal and optical field margins do not coincide; the retinal field margin lies between 9° and 12.5° inside the optical field margin. This results in a blind sector at the margin of each eye's optical field. Consequently the visually functional retinal binocular field widths are significantly narrower than the optical binocular fields. When retinal binocularity is abolished by eye movements, optical binocular fields are still retained. Thus, estimates of binocular overlap based only upon the appearance of the pupils will be erroneous. The comprehensive nature of vision beneath the bill is probably closely associated with the herons' visually guided, stealthy, foraging techniques, which result in the single-strike capture of mobile, highly evasive prey.

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