Fishermen of the Sky

A “Cliff Notes” Story

July 2006

Visitors to the park may have noticed recently an increasingly common site along the Palisades: birds of prey soaring high above the Hudson River. Decades of steep decline in their numbers — the result of the effects of pollutants such as DDT — make their reappearance a heartening event. Peregrine falcons have returned to the cliffs, and can sometimes be heard calling out from their nests. The occasional bald eagle can be seen during fall migration and into the winter, although they are more common farther north on the Hudson.

Perhaps one of the more interesting birds of prey that can again be seen along the Palisades in greater numbers is the osprey. The osprey (Pandion haliaetus), known also as the fish eagle, fish hawk, and sea hawk, is uniquely different from other raptors. The osprey can be found living near lakes, rivers, and oceans on every continent except Antarctica, yet there is almost no variation within the species, so that ospreys that live in different areas of the world are remarkably similar to one another.

The osprey weighs only about four pounds, and its body length ranges from 22 to 25 inches, with a wingspan of four and a half to six feet. Its appearance is similar in some respects to that of the bald eagle, but the osprey can be identified by several distinct features. The osprey has a dark brown back and a white belly and head, and is distinguished by a dark stripe that covers its eyes and extends back across the head. Female ospreys typically have a dark brown “necklace” as well. The osprey’s wing is long and narrow, and angled sharply back at the carpal joint (their “elbow”).

Some of the osprey’s most interesting and distinctive physiological features, however, are not ones that are readily recognizable from a distance. The osprey, whose diet consists almost entirely of fish, has many physical attributes that make it an ideal fisherman. To catch its prey, an osprey must swoop down into the water to pluck up a fish, and oftentimes will plunge almost entirely into the water in the process.  The osprey’s feathers are oily to keep them waterproof, and its nasal valves close underwater. Its feet are likewise specially adapted for fishing. Unlike other raptors, which have one back and three front toes, one of the osprey’s front toes is reversible, and can be brought to the back so that the bird can have two toes on either side of its foot. This enables the osprey to carry fish more securely in the air and to align the fish so that it is pointing forward for more aerodynamic transportation of its meal. In addition to the reversible toe, the osprey’s feet are covered with sharp barb-like scales, called spicules, which allow the bird to grip a fish even more securely during flight. (These advantages are accompanied by a disadvantage, however: sometimes an osprey will grab a fish in the water that is too heavy for it to carry, but the vice-like grip provided by the spicules prevents the osprey from releasing its catch, and the bird can be pulled into the water and drowned.) Although the osprey’s beak is less than an inch long, it is razor sharp and curved to allow it to make short work of the flesh of its catch.

Assisting in the ospreys’ fishing abilities is their vision. Like most predators, raptors have forward-facing eyes that allow them to determine distance more effectively. Birds of prey have vision that is up to eight times more precise than that of the human eye, giving them the sharpest vision among all organisms on earth. Their large eyes allow in a maximum amount of light and thus a larger image with better resolution (that is, a more detailed, sharper image). Ospreys, like other raptors, have two foveae. The fovea is the area of the eye with the highest density of visual receptors, and consequently the area of the eye that sees images most clearly. Raptors have both a central and a lateral fovea, meaning that they can see clearly with both the middle and the side of their eyes (imagine being able to see something out of the corner of your eye as clearly as something right in front of you!).

When fishing, an osprey hovers above the water until it spies a school of fish. Most often, the osprey will then dive through the air at a sharp angle — at times reaching speeds up to 80 miles per hour — hitting the water feet first. Other times, the osprey will swoop down to the water more gently to pluck out a fish. When it emerges from the water, an osprey will shake the water off itself in mid-air, and then align its catch so that it is facing forward. Ospreys make a catch 75 percent of the time — a truly impressive success rate for a predator (or for a human fisherman!). Once in the air with a fish, however, they can fall victim to “pirates” like bald eagles. When this happens, the pirate bird will attack the osprey until the osprey is forced to drop its dinner to free its talons to defend itself.

Ospreys nest alongside large bodies of water in trees, atop telephone poles, and, in many areas, on man-made platforms designed for them.  They often reuse old nests when returning to their summer home after their winter migration. When mating, the female chooses her male partner, basing her decision on the quality and location of his nest. (These nests can weigh up to half a ton and include items as disparate as sticks, hula-hoops, rag dolls, and toy boats!) Ospreys usually mate for life, but if the pair is unsuccessful in mating they will “divorce.” Young ospreys begin to mate at age three, often returning to the area where they were hatched to find a mate and breed. In the northeastern United States, eggs are laid in the nests between the months of April and June. The female osprey will almost always lay three brown-blotched whitish or pinkish eggs several days apart. The female will incubate the eggs for a period of 32–33 days, during which time the male hunts for both himself and his mate. The chicks hatch in the same order in which the eggs were laid, and older chicks dominate and receive preferential treatment, particularly if food is scarce. Following the hatch, it is mostly the female that continues to care for the young, although occasionally the male will take over, allowing her a chance to hunt. The female feeds her young small chunks of meat that she tears off a fish with her beak. The young, whose markings already look like those of adult ospreys, will fledge not quite two months after hatching, although it is not until they have been flying for about a month or so that they will leave the nest. If they are taking too long, the parents will begin to withhold food in order to encourage the fledglings to set out on their own and hunt for themselves.

Although they do not breed here, ospreys can be seen in the park, especially during their fall migration, beginning in late August and continuing through October. An osprey plunging itself into the river to emerge with a fish in its talons is one of the more stirring sights that a visitor can see here. Happily, it has also again become a relatively common one.

– Emory Davis & Katie Weiss –