by Kurt A. Anderson, Joshua M. Unghire and John Coluccy, Ph.D.
As dawn creeps over the winter horizon, a pensive hunter crouches in the blind awaiting the morning's first flight. Whistling wings soon pass overhead but offer little in the way of visual confirmation. Later, when the sun is high enough to shroud the frosty marsh in steam, duck after duck flares just out of range, frustrating the hunter and tarnishing an otherwise excellent morning afield.
Sadly, the hunter had gone to great lengths to ensure concealment. The boat and blind were adequately covered. Waders, coat, and hat had the latest camouflage pattern. Even the dog had a camo vest. But what the hunter believed to be effective concealment was viewed very differently by the ducks. A brief look at how birds actually see may explain why situations like this occur countless times during duck season.
The Avian Eye at a Glance
A bird's life is a high-speed aerial one. Therefore, sight is usually a bird's most important sense. Hearing would come second with smell and taste a very distant third and fourth, respectively. Birds use their large, prominent eyes to search for food, detect predators, and engage in complex and colorful courtship displays. Birds can see objects in fine detail two-and-a-half to three times farther away than people can, and their spectral sensitivity, which spans from near-ultraviolet (UV) to red, is far beyond that of humans.
Several unique features of the avian eye provide enhanced visual acuity. First, contraction of two powerful eye muscles allows birds to control the curvature of the cornea and lens, increasing the refractive power of both. In contrast, people can affect only the curvature of the lens. Remarkably, this ability enables diving ducks to see with the same refractive index as water, resulting in a picture-perfect view below the surface.
The second adaptation unique to avian vision is a highly developed retina. In daylight, it is the most capable of any animal. A large number of color-receptive cones within the eye allow birds to form sharp images regardless of where light strikes the retinal wall. Additionally, blood vessels in the avian eye are concentrated in a single retinal structure, known as the pecten, as opposed to being embedded throughout the retina as in the human eye. This offers less visual interference and provides greater sensitivity to motion. For example, people see fluorescent light as a constant, steady beam, but birds see a series of discrete flashes like a strobe light. This may result in a heightened ability to see the flickering of stars, which may help birds identify celestial cues and orient themselves during nocturnal migrations.
The third feature of avian sight is a richness of color perception. Like humans, birds are able to depict reds, yellows, and blues with clarity, but the images are much more vibrant than what we see. The eyes of birds have an extra set of cones designed specifically for capturing UV radiation, increasing sensitivity to light in what is known as tetra-chromatic vision. Since UV light is the first and last of the day, this enables birds to function fully at dawn and dusk when risk of predation is less than during midday. Some waterfowl food items even emit UV light and direct the birds' feeding behavior (see sidebar). In addition, differences in the UV intensity of feathers influence mate selection, dominance, and reproductive success. The feathers of each species differ dramatically in color and hue and improve with age. For example, the chocolate brown hoods of drake pintails reflect different amounts of UV-A light and separate juvenile drakes from mature birds.