Visual fields in short-toed eagles, Circaetus gallicus (Accipitridae), andthe function of binocularity in birds

Visual fields were determined in alert restrained birds using an ophthalmos copic reflex technique, The region of binocular overlap is relatively small : maximum width of 20 degrees occurs approximately 15 degrees below the hor izontal, and the field extends vertically through 80 degrees with the bill tip placed close to the centre, Monocular field width in the horizontal pla ne is 139 degrees, and the field is asymmetric about the optic axis, The cy clopean field extends through 260 degrees, and the blind area above and beh ind the head reaches maximum width of 100 degrees close to the horizontal. At the frontal margins of the monocular field the retinal and optical field s do not coincide; the retinal field margin lies approximately 10 degrees i nside the optical margin. This gives rise to an apparent binocular field th at is twice the width of the functional binocular field. Interspecific comp arisons show that the binocular field of Short-toed Eagles is similar in sh ape and size to those of bird species that differ markedly in phylogeny, ec ology, foraging technique, and eye size. This suggests that these relativel y narrow binocular fields are a convergent feature of birds whose foraging is guided by visual cues irrespective of whether items are taken directly i n the bill or in the feet, as in eagles, and irrespective of the size and s hape of the monocular and cyclopean visual fields, It is argued that binocu lar vision in birds results from the requirement for each monocular field t o extend contralaterally to embody a portion of the optical flow field whic h is radially symmetrical about the direction of travel. This is in contras t to functional explanations of binocularity, such as those concerned with stereopsis, which present it as a means of extracting higher order informat ion through the combination of two monocular images of the same portion of a scene.