Conjugate and disjunctive saccades in two avian species with contrasting oculomotor strategies.

We have recorded with the magnetic search coil method the spontaneous saccades of two species of predatory birds, which differ in the relative importance of panoramic and foveal vision. The little eagle (Haliaetus morphnoides) hunts from great heights and has no predators, whereas the tawny frogmouth (Podargus strigoides) hunts from perches near the ground, is preyed upon, and frequently adopts an immobile camouflage posture. We find that both birds spend most of the time with their eyes confined to a small region of gaze, the primary position of gaze; in this position, the visual axes are much more diverged in the frogmouth than in the eagle, thereby giving it a larger total field of view at the expense of its binocular field. Both species, however, can align their visual axes, thereby gaining a binocular view of targets in the frontal visual field. The eagle makes saccades 10 times as frequently as the frogmouth, and in most of its saccades the eyes move in the same direction, although the eye movements are generally not conjugate; in most frogmouth saccades, on the other hand, the eyes move in opposite directions. We discuss these phenomena in terms of their saccadic mechanisms and their possible adaptiveness. We also report two incidental observations: in certain circumstances, the frogmouth shows no sign of having a vestibulo-ocular reflex, and the eagle, although the quintessential foveate animal, shows extreme directional asymmetries of monocularly evoked optokinetic nystagmus.