Saturday, 11 February 2017

Some notes on the acuity of eyesight in raptors, and thinking about their perception of images in relation to our own.

In reference to a number of biological papers that I have recently been reading and thinking about, I thought it prudent to make some notes to help me build the all-important, yet elusive point of view of the Peregrine, and here I am talking about that same Peregrine that JA Baker chose to observe for over 10 years, back in the 1950s and 60s.

My source for these notes have come mainly from an American group, "The BioMedia Association" who have placed some valuable material directly online (the details of which can be found in the References section below). This has been supported by work from Vance A.Tucker, who interestingly, being based at Duke University, is located in North Carolina, US: whereas The BioMedia Association is based in South Carolina, US. I wonder if there is a link? Anyway onto my notes…

Raptors or birds of prey, including the eagles, hawks and falcons have up to eight times more clarity of sight than the human eye (with 20+ 20+ vision). A golden eagle, for example, can see a rabbit from a distance of over one mile. The Raptor’s sharp vision is also related to very specific and peculiar feeding and flying patterns. (Biomedia Assoc (2013).

Vision is necessary for flight.  Without vision, birds cannot fly as they are unable to make directional reference sufficiently for both take off and sustained flight. Predatory evolution and feeding habituation, particularly on smaller rodents, vertebrates, invertebrates and aquatic species necessitated a more efficient method of capturing such prey. Favouring high vantage points, flight (initially through gliding) became the norm for these raptors; they also needed abilities to see at close range, and they needed to maintain focus at high speeds when in pursuit. The most successful birds of prey in any population were those with eyes better adapted to these demands. Over thousands of generations, natural selection led to populations of predator birds with greater visual capabilities. Biomedia Assoc (2013), Tucker, V.A. (2000).

According to Tucker, there are two regions of the retina found in raptors, that account for the significant increase in accuracy of vision. These are discussed as the primary fovea, or the shallow fovea, where the receptors of the retina, while pointing forward within the skull, overlap at approximately 15° to the right or left of the head axis. This shallow fovea is not too dissimilar to that found in other animals. Whereas the secondary, or deep fovea, again pointing forward in the skull within the line of sight, also has a range of approximately 45° to the right or left of the head axis. Furthermore, the central beak of the bird, lying symmetrically on either side of the head axis, creates a reference line. (Tucker, V.A.(2000)).

How do predatory birds see more sharply than us?
 All raptors characteristically have large eyes. With a greater opportunity to allow light to enter into the eye chamber (e.g. as a ‘camera’), they also allow for a larger image to be projected through the lens into the rear of the eye chamber. If a retinal image is spread over a greater number of visual cells; it necessarily follows that there will be higher resolution to the detail in the picture.
Bird's eyes are so huge that a significant portion of the skull is devoted to them, allowing only limited room for the brain. Biomedia Assoc (2013).   Nevertheless, memory, recognition and cognition play an important part in the successful functioning of the organism.
A bird in flight, hunting small prey must not only be able to distinguish how far away the prey is but also its size, shape, position, and motion. In response to these challenges, raptorial birds also evolved precise accommodation and binocularity. (2013).

To determine the size and distance of prey, predators rely on memory as well as visual information. Through recognition (memory) it is possible to determine (through recall) how large the prey animal is, and then judge its distance based on the size of the image available on the eye’s retina. A clear image of the prey, no matter what its distance is also critical for the correct cognition of it. (2013).
The eye automatically focuses at a variety of distances using a natural neuromuscular adjustment called accommodation. In this process, microscopic ciliary muscles surrounding the eye alter the curve of the lens within the eye, to allow focus on objects that are far or near. Raptor’s eyes have exceptional capabilities for accommodation. Thus, as a potential prey moves closer or farther from the eagle or hawk, the predator's eyes remain focused by rapidly changing the lens curvature accordingly. (2013)

Raptors have front facing eyes. Therefore binocular vision is similar to our own. In binocular vision, the fields of view of the left and right eye overlap. Whereas, creatures with eyes on the sides of their head (especially prey animals) have low binocularity (what each eye sees overlaps very little) but high periscopicity (each eye has a full field of view) to evade predation by early cognition of threat. Biomedia Assoc (2013).

The right-eye and left-eye visual fields of a hawk overlap about 90 degrees, (in human vision, this overlap is about 120 degrees). A further adaptation in raptors - the cornea and lens are angled toward the beak to increase the overlapping region further.
Binocularity allows for stereoscopic vision, which in turn permits the determination of distance. When an organism compares the slightly different images from the right and left eye, the brain is capable of automatically determining the distance to the object. Raptors, with their greater amount of visual field overlap, have the most significant abilities to use binocularity to develop a sharp, three-dimensional image of a large portion of their view. Biomedia Assoc (2013)

The Deep, or 'Second' Fovea:
In birds that need accurate distance vision, (i.e.: birds of prey and some other species), a second fovea evolved in the lateral part of the retina. The fovea is a small region of the retina where the concentration of rods and cones is highest and therefore vision is at its sharpest. Raptors, with their broad binocular field of view, have both a central and lateral fovea. As a result, a substantial proportion of their visual field projects to the most visually sensitive parts of the retina. Tucker, V.A, (2000) & Biomedia Assoc (2013)

Another unique structure found in almost all bird species eyes is the pecten. Pecten is a thin, greatly folded tissue extending from the retina to the lens. Predatory birds such as eagles and hawks have the largest and most elaborate pecten. This unique structure supplies nutrients and oxygen throughout the vitreous humour of the eye, thereby reducing the number of blood vessels in the retina. With fewer blood vessels to scatter light entering the eye, raptor vision has evolved to be the sharpest image known among all organisms. Biomedia Assoc (2013)


In thinking about my readings, I undertook a few weeks ago (concerning ancient Egyptian mythology). Concerning the god of the sky "Horus", I find it quite surprising that those ancient Egyptian thinkers and particularly the "priests" or shamanic leaders chose the Peregrine Falcon as the symbol of the God of all gods, and great overseer. How did they know, over 5000 years ago, that this simple bird of prey was indeed, not simple at all!

It is fascinating that science only over the past hundred or so years is now able to prove the observations that those ancient Egyptians were able to recognise 5000 years earlier. The myths of Egypt's heritage hold Horus, (usually depicted in human form but with the head of the Peregrine Falcon) as the highest deity, and it is from their original notions that the all seeing eye has been reinterpreted, indeed re-conceived, throughout the ages. The concept extends to almost all the major religions.

My determination to stay with the original project ideas seem to be continually reinforced.


Notes recorded from material provided by
 BioMEDIA ASSOCIATES, LLC (2013), Beaufort, South Carolina, US.
Tucker, V.A. (2000) “THE DEEP FOVEA, SIDEWAYS VISION AND SPIRAL FLIGHT PATHS IN RAPTORS” Department of Biology, Duke University, Box 90338, Durham, NC,
in The Journal of Experimental Biology 203, The Company of Biologists Limited, Great Britain  (pp 3745 – 3754).

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