Bird’s Eye View

 With a wingspan of up to over eight feet and eyesight over eight times stronger than that of a human, the Golden Eagle is one of the most feared hunters in the wild. They possess 20/5 vision, and they can spot an object from 20 feet with the same clarity a visually unimpaired human can from 5 feet [1]. Planet Earth II, episode 2 portrays a golden eagle flying through the Alps foraging for prey. Upon spotting a fallen deer from miles away, the bird locates its food. This exceptional visual acuity allows these birds of prey to hunt for even the tiniest animals from hundreds of feet in the air.

One reason behind the incredible eyesight of birds of prey lies in the density of their photoreceptors and cones. Birds of prey contain a deeper fovea to accommodate a greater number of photoreceptors and cones, leading to greater visual sensitivity and depth of perception [2]. These deeper fovea also serve as a convex lens to magnify the retinal image, further enhancing the eyesight of these birds [2]. In addition to the traditional adjusting of the lens (accomodation), which mammals possess, birds of prey also possess an adaptation termed corneal accommodation. This allows the cornea shape to adjust simultaneously with the lens in order to maintain visual sensitivity as they fly at high speeds to find prey [3].

A common misconception is that the evolution of visual adaptations that led to specialized eyes in birds of prey developed over one generation. Several generations with the evolutionary forces of natural selection, migration, gene flow, and mutation are necessary for this type of eyesight to evolve in birds of prey from their ancestors. Birds of prey must adapt to multiple environmental pressures, such as differing food sources, vegetative coverage, altitudes and predators. The food sources commonly exploited by the birds of prey are small animals, especially rodents [4]. To survive in these diverse ecological niches with differing food sources, these birds have developed the ability to see ultraviolet light and detect more colors than human eyes can [5]. This ability allows them to visualize and follow traces of urine left behind by mice and rats, which can be seen in the ultraviolet range [5].

by Jay Narula, Jillian Grace, Monisha Mistry and Bryan Oh

Planet Earth II, Episode 2, starting at 9:30

References

1. Jones, MP, KE Pierce, and DWard. 2007. Avian vision: a review of form and function with special consideration to birds of prey. Journal of Exotic Pet Medicine 16(2):69-87.
2. Ruggeri, M JCMajor, C McKeown, RW Knighton, CA Puliafito and S Jiao. 2010. Retinal structure of birds of prey revealed by ultra-high resolution spectral-domain optical coherence tomography. Invest. Ophthalmol. Vis. Sci. 51(11):5789-5795.
3. Levy B and JG Sivak. 1980. Mechanisms of accommodation in the bird eye. Journal of comparative physiology 137(3):267-72.
4. Remsen Jr JV and SK Robinson. 1990. A classification scheme for foraging behavior of birds in terrestrial habitats. Studies in Avian Biology 13: 144-160.
5. Ruggeri M, JC Major, Jr., C McKeown, RW Knighton, CA Puliafito and S Jiao. 2010. Retinal structure of birds of prey revealed by ultra-high resolution spectral-domain optical coherence tomography. Investigative Ophthalmology & Visual Science 51(11):5789-95.