Osprey (fish hawk)
Osprey, Pandion haliaetus, is a member of the Family Pandioninae. There is only one species of Osprey found worldwide. This fascinating raptor plunges into water for fish, the only raptor that does. Ospreys are medium-large raptor, measuring 21-24.5 inches long with a 4.5 to 6 foot wingspan. Coloration is black above and white below. The head is mostly white with a black streak through the cheeks. The underside of the wings and tail is striped black and white with a large black spot at the bend in the wing. Weight is about four pounds. As with most raptors, the female is usually larger than the male.
The breeding habitats in North America are rivers, lakes, wetlands and coastal marshes. Adult male Ospreys arrive on their nesting territories a few days before the females. This nest is in the general area of the nest from which they fledged and was built or used the year before. Osprey will build nests on dead trees, rock outcrops, buildings, buoys, electric towers, pier pilings, and man-made platforms, i.e. almost anywhere near water. Osprey nests are a bulky mass of sticks often five feet in diameter and two to seven feet thick. They seem to prefer nesting sites with clear visibility and an abundant food supply. Osprey seem to form loose colonies. The number of nests in close proximity and the distance between nests seems to be determined by food abundance. Soon after his arrival at the nest, the male starts sky-dancing over the nest. During this aerial display the male flies sharply up rapidly beating his wings and often carrying a fish or nesting material. At a height of several hundred feet the bird hovers with tail fanned and talons dangling. He then dives down to varying distances and quickly ascends to repeat the hover several times, often uttering a creee or cheeerk call. The sky-dance display is preformed frequently before the arrival of the female and continued less frequently after her arrival. The sky dance seems to have two functions, a territorial display and to advertise for a mate. The return of both birds to a nest appears to be a function of territorial fidelity more than pair bonding. The arrival of the female is followed by a period of courtship that can last for up to three weeks. During courting the pair spends time on the nest together, as well as mate-feeding and copulating. Soon after the female’s arrival, the male starts to bring all of her food. The female may take the food to a perch to eat, returning after her meal with material to line the nest. Copulation may take place after the mate feeding ritual and is often preceded by a mating display by the male, female or both. The female’s display posture has slightly drooped wings, horizontal body and tail pointed up and to one side. The male might turn his back to the female, and spread his wings out and down with tail depressed.
Incubation begins when the first egg is laid. Subsequent eggs are laid one to three days apart; clutches have 2-4 eggs. The female usually takes on most of the responsibility of incubation, seldom leaving except to feed. The male then takes over incubation until her return. Incubation takes from thirty-four to forty days.
The first chick to hatch has an advantage over the siblings which hatch a day or two later. The first chick grows quickly and can dominate the nest. In years when the food supply is low, the dominant first chick can usurp the available food supply to the peril of the smaller siblings. This insures that at least one of the year’s brood might survive. The chicks are brooded by the female for approximately ten days. The young are too large to fit under her by this time. She will continue to protect them by covering the young with her wing.
Division of Labor
The male, dedicated to providing for his family, does all of the hunting until the chicks are six weeks old. The male delivers the fish to the female on the nest who tears off pieces to feed to the young. At three to four weeks of age the chicks start to exercise their wings by holding onto the edge of the nest and flapping their wings. Mom then moves to a near by perch to guard the nest. The female may leave the nest to hunt when the chicks are six weeks old. The young start to feed themselves at this time.
At seven to eight weeks of age the young will take their first flight. They spend their days practicing flying and perching near the male’s feeding perch calling when the male returns with food. Two weeks after fledging the young will start to follow the male on hunting trips. Four to eight weeks after fledging the young Osprey will start to emulate their parents and begin to hunt fish on their own.
Upon spotting a fish near the surface of the water, the Osprey will hover almost directly above and then dive almost vertically towards the fish. At the last moment before impact with the water, the Osprey thrusts its talons forward to snatch the fish. The Osprey is often fully immersed in the water and must struggle to break free of the water and lift the heavy fish into the air. The Osprey’s talons are specially adapted to allow one of its three front toes to bend backwards so that there are two claws pointing forward and two in reverse on each foot. This helps to secure a good grip on a slippery fish. Scrupulous in maintaining its aerodynamic profile, the Osprey adjusts the fish in its grip as soon as it clears the water so that the fish’s head is forward and the tail aft. This fishing technique is quite different from that of the Bald Eagle that swoops almost parallel to the water surface to grip a fish as it passes over. The Eagle does not bother with adjusting the fish to an aerodynamic position.
In the fall, young Osprey from the Northern States will drift south following the coast through Central America to northern South America. These young Ospreys do not return to North America the following spring. They remain behind in South America while the adults head north. If they survive the trip south, a year and a half in South America, and a return trip north they will return to their home territory to attract a mate and build a nest. The young pair will not successfully breed at the nest until they are three years old. The Osprey populations that nest in North-Eastern North America migrate along the Atlantic coast, crossing the ocean along the West Indies to winter in northern South America. Osprey from the southern states need not migrate because they have a year round food supply. They tend to stay put and defend their territories.
Threats to survival
The Osprey like the eagles and falcons suffered a major decline in population due to the ingestion of DDT pesticide derivatives in its food supply. These poisons collect in the fatty tissues of fish and other animals and are not expelled from the body. Raptors are at the top of the food chain meaning that their bodies collected and retained the poisons accumulated by all the life forms from single celled creatures to the creatures that ate them and so on up to the fish that were consumed by the Ospreys. The concentration of toxins was amplified in the process. These toxins cause Ospreys and other raptors to lay eggs that have thin shells that break during incubation. The United States banned the use of DDT and regulated other pesticides allowing a significant recovery of Osprey, Eagles, and other birds of prey. These pesticides are still in use in other parts of the world so the threat of toxins in the world food chain remains a threat.
The sites which Osprey select for nest building are exposed to weather and obvious to predators. Storms can wipe out exposed nests and a host of predators from raccoons and opossums to crows, ravens, other hawks, eagles and owls can raid an undefended nest. Due to their preference for exposed sites, Osprey often select the cross bars of power poles as nest locations. This conflict with power utilities can lead to short circuits when rain soaked nest materials bridge insulators. The birds themselves can be electrocuted if their bodies touch a live wire and a grounded part of a support structure.
Travels of a Montezuma Osprey
A former female osprey on the North Spring Pool was captured and fitted with a radio transmitter that could be monitored by satellite on June 20, 2001. She left the refuge on August 26 and followed the Appalachian Mountains south to arrive in South Florida on September 6. She then crossed the Caribbean to Cuba and on to the Dominican Republic. Her crossing from the Dominican Republic to the Islands off the coast of Venezuela took two days on the wing, arriving in Venezuela on Sep 18. She spent several weeks slowly crossing Venezuela and then powered south to arrive in the heart of Brazil on the Amazon River on October 9. Unfortunately, the tracking of the return flight was interrupted by a transmitter failure, but the signal was picked up again in 2002 as she arrived back in the United States along the Appalachians. More information about tracking the Osprey can be found at http://www.fws.gov/r5mnwr/lotw/index.html.