Eye on the Outdoors

World of a fish hawk is in the eyes of the beholder

Bob Ballantyne

POSTED: July 1, 2010

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Fact Box

Natural Facts
The uniquely shaped drift boats used on many Western rivers are called Mackenzie boats and are named for the longest river in Canada. The river, in turn, was named for 18th-century Scottish explorer, Alexander Mackenzie.
Some biologists believe that eagles and ospreys actually have three fovea per eye.
Bald eagles mate for life. Male ospreys, however, often pair-bond just for one season, but are loyal to the nest site for many years.
The presence of blood vessels in our retinas is believed to interfere slightly with vision.
Unlike the bald eagle, the osprey can hover in flight, which is why the military has named the V-22 tilt-wing aircraft the “Osprey.”
The pectin in an eagle’s or osprey’s eye is a piece of convoluted tissue that extends from the retina to the lens, and is filled with capillaries.
The eye black used by football players is often a mix of beeswax, paraffin, and carbon.

Ipicked up my fly rod and my fishing vest from the rear seat of the drift boat and scrambled forward to the upturned bow of the craft. There I could lock my legs into the knee braces built into the framework and stand up while fishing. Within a few moments, the oarsman for the day, a Montana friend of mine, pushed off from the McConnel Access to the Yellowstone River just outside the famous national park, and we were on our way.

I have made this half-day drift many times. This trip, however, would turn out to be the best float ever for me on the river, as measured in terms of number of fish caught and released. In fact, it was almost a grand slam in Yellowstone jargon, for three species of trout – brown, rainbow, and cutthroat – along with some whitefish, were lured to the net by flies at the end of the tippet. Only a brook trout, rare for this river, failed to take the trickery.

The standard technique for such fishing is to cast the dry fly to what is called “the seam,” a clearly perceived natural line in the water where the shallowness of the river near its edge produces water slower than the faster run of the main flow. Trout lie in this slower water awaiting the passing of some floating insect on the moving water. An imitation of such a meal, if drifted properly, is what may lead to success when drift-boat fishing.

While drifting along one such section of the bank, the placid sounds of the river were interrupted by a squawking from above. As it turned out we were drifting beneath the nest of some ospreys, and a nest-bound offspring of its parents circling above was apparently protesting our presence, or perhaps its calling was to its parents, begging for a meal.

Ospreys and bald eagles have many unique adaptations for their life as fishers. The structure of their eyes is a major aid in fishing. Both species, for example, have a featherless area around their eyes, although it is much more pronounced in eagles. Feathers, of course, would interfere with their vision. The bald eagle also has a bony structure called the superciliary ridge in that featherless area. That structure gives the eagle its fierce look. It is absent in osprey.

One field mark of an osprey, however, is the dark band that runs around its eyes and along the sides of its head. This helps reduce the glare from a bright sun while the osprey fishes, much like the eye black used under the eyes of football players.

An eagle’s eyes are larger than ours, even though their head is much smaller. Our eyes, like those of other mammals and birds, have a section of the retina called the fovea where the image-detecting cells called rods and cones are concentrated. Eagles and osprey have five times as many such cells per square millimeter in their fovea. In fact, they have a second fovea, and such traits give them much sharper vision. Oddly, the retina of these birds of prey has no blood supply as does ours. Instead, they have a structure called a pectin, a different type of organ for supplying oxygen and other nutrients to the retina.

The osprey pair that circled above us that day on the Yellowstone River was not the only encounter I have had with them, or with bald eagles, over my years of fishing. One time here in Pennsylvania, a friend and I were repeatedly dived at by an osprey as we fished for brook trout behind an old beaver dam on a Pocono stream. And in the same year of the float trip described above, I fished a section of the Gardner River in Yellowstone National Park for two hours, the entire time under the watchful eye of a bald eagle sitting high in a snag above the river.

That eagle, apparently, was hoping for a fish to be released weakened enough that it could dive upon it. Eagles, it seems, will also feed on dead fish, while ospreys, it has been reported, seldom do. Also, ospreys can dive deeply into the water in pursuit of their prey, while bald eagles do not submerge as deeply.

And as to my own visual acuity while fishing both from a drift boat or when wading the waterways of Yellowstone country, Pennsylvania, or elsewhere, I do as many other anglers do and use polarized sunglasses. They are an aid to cutting through the glare to perceive my quarry in the clear waters that I gladly share with both the osprey and the eagle.