Laterality in avian vigilance: do sparrows have a favourite eye?

Visual scanning for predators is one of a bird's most basic means of avoiding predation while feeding, and the detection of an approaching predator may present a difficult visual task. If birds have a dominant hemisphere for processing visual information, then given the nearly complete optic nerve crossover in the avian brain, the eye contralateral to that dominant hemisphere should be favoured in antipredatory vigilance. In short, vigilant birds should have a favourite eye. We conducted an experiment that examined laterality in eye use during the antipredatory scanning of wintering emberizid sparrows: dark-eyed juncos, Junco hyemalis, and American tree sparrows, Spizella arborea. Free-living birds were allowed to feed parallel to a visual obstruction, thereby forcing them to choose which eye to orient away from the visual obstruction and towards possible approaching predators. Juncos oriented themselves such that their right eye was oriented outward more than would be expected by chance. Tree sparrows had a marginally nonsignificant tendency to favour the left eye. The mechanics of the scanning process itself in these birds favoured one eye over the other. Both species turned their heads towards the visual obstruction when their preferred eye was facing away from the obstruction more often than would be expected by chance (right eye in juncos, left eye in tree sparrows); such behaviour allowed for a preferred-eye view of the area directly opposite protective cover. These results suggest that laterality exists in avian vigilance, and that laterality may differ even in closely related species. (C) 2001 The Association for the Study of Animal Behaviour.