Visual fields in the Black-crowned Night Heron Nycticorax nycticorax: Nocturnality does not result in owl-like features

Compared with diurnally active species, the eyes of nocturnally active herons (Ardeidae) are relatively larger and more widely separated. These features are found in comparisons between the nocturnally foraging Black-crowned Night Heron Nycticorax nycticorax and the diurnally active Cattle Egret Bubulcus ibis. Casual viewing of the Black-crowned Night Heron gives the impression of a somewhat owl-like appearance, with an apparently wide degree of binocular overlap. Visual fields and eye movements were determined in two alert, restrained Black-crowned Night Herons with the use of an ophthalmoscopic technique. A wide degree of binocular overlap was not confirmed, and the Black-crowned Night Heron's visual field closely resembles those of diurnally foraging herons (Western Reef Heron Egretta gularis schistacea, Squacco Heron Ardeola ralloides, Cattle Egret). The Black-crowned Night Heron's binocular field is vertically long and narrow (maximum width 22°), with the bill placed approximately at the centre. Monocular and cyclopean field widths in the horizontal plane equal 171°and 320°, respectively. Retinal binocular overlap can be abolished by eye movements. There is a blind sector (10-13°wide) at the margin of each eye's optical field, and this results in the functional retinal binocular field being much narrower than the optical binocular fields. It is these blind sectors which give rise to the appearance of a much wider binocular field. The visual field characteristics of this heron species may be best understood in relation to the foraging technique of capturing agile, evasive prey directly in the bill. The comparatively large size of the Black-crowned Night Herons' eyes may be associated with activity over a wide range of natural light levels but does not give rise to binocular fields larger than those of diurnal heron species.