An insight to raptorial vision

In comparison to humans, birds generally have larger eyes considering their body and head size as the eyes can take up to 50% of the skulls volume (Jones, Pierce & Ward 2007). This means they depend on a more visually active lifestyle and that they use their eyesight to not only find food but also to avoid being predated, as well as observing their environmental surroundings (Jones, Pierce & Ward 2007, Garamszegi, Møller & Erritzøe 2002).

Fig 1. Orientation and visual field in avians from (Martin 2009)

With a focus towards raptorial adaptations, the orientation of the eyes in diurnal raptors is relatively laterally orientated, having a better monocular vision but narrow binocular vision, whereas forward orientated owls have a greater binocular visual field but lack posterior vision (Fig.1) (Iwaniuk, Hall & Wylie 2008).Also depending on what raptor is being observed there will be differences in the eyes size and shape (Jones, Pierce & Ward 2007). 

The eye shapes seen in birds of prey mainly consist of round and tubular shaped eyes (Fig 2.) (Sturkie 1987). These eye shapes are characteristic of diurnal and nocturnal raptors respectively, as diurnal raptors require the ability to see long distances in higher definition, in contrast to owls and with keener eyesight from their concave ciliary region (Jones, Pierce & Ward 2007, Sturkie 1987).

Fig.2 attained from Sturkie (1987 p.39)

But aside from orientation and exterior morphology, internal structures such as the retina and fovea will be discussed.
The retina (Fig3.) contains rods and cones, which allows diurnal and nocturnal raptors to see in their environment (Jones, Pierce & Ward 2007, Sturkie 1987). These raptors are cone dominated and rod dominated respectively, as cones have photoreceptors (Fig 5.) allowing them to filter out different wavelengths of light depending on the oil droplet they contain, whereas rod photoreceptors in low light environments discern shapes and movements (Jones, Pierce & Ward 2007, Sturkie 1987). 

The fovea (Fig 4.) is characteristically deep in diurnal raptors and is, in fact, something that most taxa lack, but raptors have two foveae, being the area that aids in the acuity of vision, this allows diurnal raptors see on more than a single plain stationary and whilst hunting (Jones, Pierce & Ward 2007, Sturkie 1987).  

Fig 3. Obtained from Glasser & Howland (1996 p.479)

Fig 4. deep (A) and shallow fovea (B) from Jones, Pierce & Ward (2007 p.77)

Fig 5. Labeled diagram of cone photoreceptor from Jones, Pierce & Ward (2007 p.77)

Reference:


Glasser, A. & Howland, H.C. 1996, "A History of Studies of Visual Accommodation in Birds", The Quarterly Review of Biology, vol. 71, no. 4, pp. 479.

Garamszegi, L.Z., Møller, A.P. & Erritzøe, J. 2002, "Coevolving avian eye size and brain size in relation to prey capture and nocturnality", Proceedings of the Royal Society of London. Series B: Biological Sciences, vol. 269, no. 1494, pp. 961.

Iwaniuk, A.N., Heesy, C.P., Hall, M.I. & Wylie, D.R.W. 2008, "Relative Wulst volume is correlated with orbit orientation and binocular visual field in birds", Journal of Comparative Physiology A, vol. 194, no. 3, pp. 267-268.

Jones, M.P., Pierce, K.E. & Ward, D. 2007, "Avian Vision: A Review of Form and Function with Special Consideration to Birds of Prey", Journal of Exotic Pet Medicine, vol. 16, no. 2, pp. 69-79.

Martin, G.R. 2009, "What is binocular vision for? A birds' eye view", Journal of Vision, vol. 9, no. 11, pp. 14-14.

Sturkie, P. D., 1987, Avian Physiology: Fourth Edition, Springer-Verlag, New York, pp. 39-45.