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Gregory R. Mann, Ph.D. {ret.}

Osprey

“Pandion haliaetus”

Unique among raptors for its diet of live fish and ability to dive into water to catch them, Ospreys are common sights soaring over saltwater & freshwater shorelines, patrolling waterways and standing on their huge stick nests, white heads gleaming. These large, rangy hawks do well around humans. It is found on all continents except Antarctica, although in South America it occurs only as a non-breeding migrant. Hunting Ospreys are a picture of concentration, diving with feet outstretched and yellow eyes sighting straight along their talons. An Osprey may log more than 160,000 migration miles during its 15-to-20-year lifetime. Scientists track Ospreys by strapping lightweight satellite transmitters to the birds’ backs. The devices pinpoint an Osprey’s location to within a few hundred yards and last for 2-3 years. During 13 days in 2008, one Osprey flew 2,700 miles from Martha’s Vineyard, Massachusetts to French Guiana, South America. Ospreys are unusual among hawks in possessing a reversible outer toe that allows them to grasp with 2 toes in front and two behind. Barbed pads on the soles of the birds’ feet help them grip slippery fish. When flying with prey, an Osprey lines up its catch head first for less wind resistance. Ospreys are excellent anglers. Over several studies, Ospreys caught fish on at least 1 in every 4 dives, with success rates sometimes as high as 70%. The Osprey readily builds its nest on man-made structures such as telephone poles, channel markers, duck blinds and nest platforms. Such platforms have become an important tool in reestablishing Ospreys in areas where they had disappeared. In some areas, nests are placed almost exclusively on artificial structures. Osprey eggs do not hatch all at once. Rather, the first chick emerges up to 5 days before the last one. The older hatchling dominates its younger siblings and can monopolize the food brought by the parents. If food is abundant, chicks share meals in relative harmony; in times of scarcity, younger ones may starve to death. A young Osprey spends the first 7 or 8 weeks of it’s life confined within the nest (or eyrie); a huge pile of sticks lined with moss, bark and grass. It will be fed primarily by the female, who tears pieces from a fish and passes them into the nest cup. At 2 weeks old, the youngsters can move around the nest and after a month they are very active preening and exercising their wings. Gradually the wing-flapping increases until they are able to lift a little off the nest called ‘helicoptering’ and then take their hesitant first flight. For at least two weeks after fledging, the young Ospreys return to their nest for food brought in by their parents. Usually the young stay in the area, close to the nest site as they improve their skill in the air before they then begin to make attempts to catch a fish for themselves.

The name “Osprey” made its first appearance around 1460 via the Medieval Latin phrase for “bird of prey” (avis prede). Some trace the name even further back to the Latin for “bone-breaker”—ossifragus. Unable to dive to more than about three feet below the water’s surface, Ospreys gravitate toward shallow fishing grounds, frequenting deep water only where fish school near the surface. Ospreys nest in a wide variety of locations and their habitat includes almost any expanse of shallow, fish-filled water including rivers, lakes, ocean shorelines, reservoirs, lagoons, swamps and marshes. Whatever the location, Osprey nesting habitat must include an adequate supply of accessible fish within a maximum of about 12 miles of the nest; open, usually elevated nest sites free from predatory mammals and a long enough ice-free season to allow the young to fledge. The Osprey is the only hawk on the continent that eats almost exclusively live fish. In North America, more than 80 species of live freshwater and saltwater fish account for 99% of the Osprey’s diet. Captured fish usually measure about 6–13 inches in length and weigh one-third to two-thirds of a pound. The largest catch on record weighed about 2.5 pounds. On very rare occasions, Ospreys have been observed feeding on fish carcasses or on birds, snakes, voles, squirrels, muskrats and salamanders. Ospreys probably get most of the water they need from the flesh of their prey, although there are reports of adults drinking on hot days. Adept at soaring and diving but not as maneuverable as other hawks, Ospreys keep to open areas, flying with stiff wing-beats in a steady, rowing motion. Primarily solitary birds, they usually roost alone or in small winter flocks of 6 to 10. Nesting Ospreys defend only the immediate area around their nest rather than a larger territory; they vigorously chase other Ospreys that encroach on their nesting areas. In breeding season, males perform an aerial “sky-dance” sometimes called “fish-flight”. With dangling legs, often clasping a fish or nesting material in his talons, the male alternates periods of hovering with slow, shallow swoops as high as 600 feet or more above the nest site. Sustaining this display for 10 minutes or more, he utters repeated screaming calls while gradually descending in an undulating fashion to the nest.

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