Visual Fields in Short-Toed Eagles, Circaetus gallicus (Accipitridae), and the Function of Binocularity in BirdsMartin G.R. · Katzir G.
aSchools of Biological Sciences and of Continuing Studies, The University of Birmingham, Birmingham, UK bDepartment of Biology, University of Haifa, Oranim, Israel
Visual fields were determined in alert restrained birds using an ophthalmoscopic reflex technique. The region of binocular overlap is relatively small: maximum width of 20° occurs approximately 15° below the horizontal, and the field extends vertically through 80° with the bill tip placed close to the centre. Monocular field width in the horizontal plane is 139°, and the field is asymmetric about the optic axis. The cyclopean field extends through 260°, and the blind area above and behind the head reaches maximum width of 100° close to the horizontal. At the frontal margins of the monocular field the retinal and optical fields do not coincide; the retinal field margin lies approximately 10° inside the optical margin. This gives rise to an apparent binocular field that is twice the width of the functional binocular field. Interspecific comparisons show that the binocular field of Short-toed Eagles is similar in shape and size to those of bird species that differ markedly in phylogeny, ecology, foraging technique, and eye size. This suggests that these relatively narrow binocular fields are a convergent feature of birds whose foraging is guided by visual cues irrespective of whether items are taken directly in the bill or in the feet, as in eagles, and irrespective of the size and shape of the monocular and cyclopean visual fields. It is argued that binocular vision in birds results from the requirement for each monocular field to extend contralaterally to embody a portion of the optical flow field which is radially symmetrical about the direction of travel. This is in contrast to functional explanations of binocularity, such as those concerned with stereopsis, which present it as a means of extracting higher order information through the combination of two monocular images of the same portion of a scene.
Prof. Graham R. Martin
School of Continuing Studies, The University of Birmingham
Edgbaston, Birmingham B 15 2TT (UK)
Tel. +44 121 414 5598, Fax +44 121 414 5619
Number of Print Pages : 12
Number of Figures : 6, Number of Tables : 1, Number of References : 51
Brain, Behavior and Evolution
Founded 1968 and continued 1968–1986 by W. Riss, New York, N.Y.
Official Organ of the J.B. Johnston Club
Vol. 53, No. 2, Year 1999 (Cover Date: February 1999)
Journal Editor: R. Glenn Northcutt, La Jolla, Calif.
ISSN: 0006–8977 (print), 1421–9743 (Online)
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