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.
Keeping a Watchful Eye
Despite their increased visual acumen, most birds are saddled with one competitive disadvantage: monocular vision (see sidebar). Avian eyes are set on each side of the head and are capable of only limited rotation toward the bill tip. Birds are therefore able to see better to the side than straight ahead. This often forces birds to observe objects one eye at a time, and the resulting image is flat and lacks accurate depth perception.
But birds have several visual adaptations that help compensate for the limitations of monocular vision. The first is an increased panoramic view. For example, mallards possess a 360-degree lateral viewing window where they can see in a complete circle at all times. People have a much narrower range of vision, which forces us to look around in several directions to piece together all of our surroundings. Bird eyes are also often set higher on the head, which increases the vertical sight plane. An extreme example is the American woodcock, which can observe not only 360 degrees laterally but also 180 degrees vertically. The benefit for birds is an increased awareness of their surroundings and a subsequent decrease in exposure to predation.
Another way birds compensate for monocular vision is rapid head movement. By moving their head rapidly from side to side, birds can observe an object with one eye from two different angles in quick succession. This creates a three-dimensional picture and greatly improves depth perception. Although difficult to see in ducks, this behavior can easily be observed in a backpedaling goose as the bird swivels its head from side to side judging the distance, location, and timing of its landing.
Monocular vs. Binocular Vision
With eyes set on the sides of their head, most waterfowl view the world with monocular vision (each eye is used separately) rather than binocular vision (both eyes view the same object at once). With the notable exception of owls, binocular vision is rare in birds. Among waterfowl, only the blue duck of New Zealand can look straight ahead. Bitterns can also do the same, but only by pointing their bill skyward.
Like a Neon Sign
The ability to perceive UV light provides birds a distinct advantage when foraging for food. The waxy surface of many fruits and berries reflects UV light, advertising their presence to hungry birds. Common kestrels hunt for voles by locating their trails visually. The urine of these small rodents reflects UV light, which kestrels are able to follow for a quick meal. Additionally, the ability to perceive urine may assist birds in avoiding potential predators. Research is currently being conducted to determine whether prairie waterfowl can detect the urine of potential predators when choosing a nesting site, thereby avoiding areas with higher predator densities.
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