The Power of the Avian Eye
The bird’s sense of sight extends beyond human capacity, and is one of the most advanced adaptations in the animal kingdom. The distinct anatomy of the avian eye lends an advantage in monitoring surroundings and spotting prey. Wide angle vision and double centers of high resolution allow the bird to take a whole scene in at once instead of piecing elements together. Songbirds and raptors can resolve details 3 times farther than humans can. Woodcocks observe their surroundings 360 degrees laterally, 180 degrees vertically.
The power source of the eye lies in the pecten, a compartment of blood vessels that protrudes off the retina. Rod and cone cells are distributed across the retina, and contain pigments that relay light information to the brain. Avian cone cells are especially dense, giving the bird the most acute daylight vision of any animal. Birds who spot moving prey have two concentrations of cone cells on the retina, called foveae. The temporal and central foveae discern between binocular and peripheral vision, respectively.
A bird’s sense of color is more nuanced than mammals because they have four types of cones, deeming their vision ‘tetrachromatic’. This color range extends into the UV spectrum, outside the realm of human vision. Birds of prey use the UV range specifically to target small mammals when hunting. An experiment by Viitala et al concluded that kestrels track voles using UV traces in the prey’s urine. The distribution of voles is unpredictable because of recurring fluctuations in population size. Kestrels overcome this setback by scanning massive fields for congregations of UV light that signal a group’s location. Recent research has unearthed multiple mechanisms for UV vision. The Common Starling preferentially feeds young that have UV-reflective skin, and the Redwing of Europe prefers berries that reflect UV light.
During migration, the bird harnesses the power of the eye for orientation. Like other migrating animals, birds monitor the sun’s position to direct their movement. The bird’s relationship to the sun is more complex; they use the directional polarization of sunlight scattered by the atmosphere. (This phenomenon, known as ‘Rayleigh scattering’, makes the sky appear blue to humans). Migrating birds tap into the earth’s magnetic field to orient themselves toward their goal. One mechanism for this sense is a magnetic compass built with photopigments in the avian eye. Sunlight excites electrons in the pigment rhodopsin, rendering the protein paramagnetic and responsive to the earth’s magnetic field.