The Osprey (Pandion haliaetus
), also known colloquially as seahawk, fish hawk or fish eagle, is a medium-large fish-eating bird of prey or raptor. However, it is not the same as a sea-eagle. It is found on all continents except Antarctica although in South America it occurs only as a non-breeding migrant. It is widely distributed because it tolerates a wide variety of habitats, nesting in any location which is near a body of water and provides an adequate food supply. It is divided into four similar subspecies.
Because the Osprey has many unique characteristics, it has been given its own taxonomic genus, Pandion, and family, Pandionidae. It is a medium-sized raptor, reaching 60 cm (24 in) in length with a 1.8 m (6 ft) wingspan. It is brown on the upperparts and predominantly whitish on the head and underparts, with a brownish eyepatch and wings.
As its other common names suggest, the Osprey's diet consists almost exclusively of fish. It has evolved particular physical characteristics and exhibits some unique behaviours to assist in hunting and catching prey.
The Osprey is 1400–2000 g (3-4.4 lb) and 52–60 cms (20–24 in) long with a 150–180 cm (5–6 ft) wingspan. The upperparts are a deep, glossy brown, while the breast is white and sometimes streaked with brown, and the underparts are pure white. The head is white with a dark mask across the eyes, reaching to the sides of the neck. The irises of the eyes are golden to brown. The bill is black, with a blue cere, and the feet are white with black talons. A short tail and long, narrow wings with four long, finger-like feathers, and a shorter fifth, give it a very distinctive appearance.
The sexes appear fairly similar, but the adult male can be distinguished from the female by its slimmer body and narrower wings. The breast band of the male is also weaker than that of the female, or is non-existent, and the underwing coverts of the male are more uniformly pale. It is straightforward to determine the sex of breeding pair, but harder with individual birds.
Juvenile Osprey may be identified by buff fringes to the plumage of the upperparts, a buff tone to the underparts, and a streaked feathers on the head. During spring, barring on the underwings and flight feathers is a better indicator of a young bird, due to wear on the upperparts.
In flight, the Osprey has arched wings and drooping "hands", giving it a gull-like appearance. The call is a series of sharp whistles, described as cheep, cheep or yewk, yewk. Near the nest, the call is a frenzied
Distribution and habitat
The Osprey has a worldwide distribution. It is found in temperate and tropical regions of all continents except Antarctica. In North America it breeds from Alaska and Newfoundland south to the Gulf Coast and Florida, wintering further south from the southern United States through to Argentina. The Osprey is found throughout Europe north into Scandinavia and Scotland, though not Iceland, in summer and wintering in North Africa. In Australia it is sedentary and found around the coastline, though only a non-breeding visitor to eastern Victoria and Tasmania. In the islands of the Pacfic it is found in the Bismarck Islands, Solomon Islands and New Caledonia, and fossil remains of adults and juveniles have been found in Tonga, where it probably was wiped out by arriving humans. It is possible it may once have ranged across Vanuatu and Fiji as well. It is an uncommon to fairly common winter visitor to all parts of South Asia, Southeast Asia including Myanmar through to Indochina and southern China, Indonesia, Malaysia and the Philippines.
The Osprey is highly successful due to its tolerance of a wide range of habitats. It may nest in any location which is near a body of water and which provides safety and an abundance of fish. Nests are generally found within 3 to 5 km of a body of water, which may be a salt marsh, mangrove swamp, cypress swamp, lake, bog, reservoir or river. Even during migration, Ospreys stay close to water, often following river valleys.
Fish comprise 99 percent of the Osprey's diet. It typically takes fish weighing 150–300 g (5–10 oz) and about 25–35 cm (10–14 in) in length, but the weight can range from 50 to 2000 g (2–68 oz). Prey is first sighted when the Osprey is 10-40 m (32-131 ft) above the water, after which the bird hovers momentarily then plunges feet first into the water. It is able to dive to a depth of one m (3.3 ft). The angle of entry into the water varies with the nature of the prey; steeper, slower dives are used when pursuing deeper, slow-moving fish, while long, quick dives are used for faster surface fish. After catching the fish considerable effort is needed to get airborne again. As it rises back into flight the fish is turned head-forward to reduce drag.
The Osprey is particularly well adapted to this diet, with reversible outer toes, sharp spicules on the underside of the toes, closable nostrils to keep out water during dives, and backwards facing scales on the talons which act as barbs to help hold its catch. Its 'barbed' talons are such effective tools for grasping fish that, on occasion, an Osprey may be unable to release a fish that is heavier than expected. This can cause the Osprey to be pulled into the water, where it may either swim to safety or succumb to hypothermia and drown. Rarely, the Osprey may prey on other wetland animals, such as aquatic rodents, salamanders, other birds, and reptiles as large as young alligators.
The Osprey breeds by freshwater lakes, and sometimes on coastal brackish waters. Rocky outcrops just offshore are used in Rottnest Island off the coast of Western Australia, where there are 14 or so similar nesting sites of which five to seven are used in any one year. Many are renovated each season, and some have been used for 70 years. The nest is a large heap of sticks, driftwood and seaweed built in forks of trees, rocky outcrops, telephone poles, artificial platforms or offshore islets. Generally Ospreys reach sexual maturity and begin breeding around the age of three to four years, though in some regions with high Osprey densities, such as Chesapeake Bay in the U.S., they may not start breeding until five to seven years old, and there may be a shortage of suitable tall structures. If there are no nesting sites available, young Ospreys may be forced to delay breeding. To ease this problem, posts may be erected to provide more sites suitable for nest building.
Ospreys usually mate for life. In spring the pair begins a five-month period of partnership to raise their young. The female lays two to four eggs within a month, and relies on the size of the nest to conserve heat. The eggs are whitish with bold splotches of reddish-brown and are about 62 x 45 mm (2.4 x 1.8 in) and weigh about 65 g (2.4 oz). The eggs are incubated for about 5 weeks to hatching.
The newly-hatched chicks weigh only 50–60 g (2 oz), but fledge within eight weeks. When food is scarce, the first chicks to hatch are most likely to survive. The typical lifespan is 20–25 years. Bubo owls and Bald Eagles (and possibly other eagles of comparable size) are the only major predators of both nests and and sub adults. Ospreys have rarely been known to be preyed on by crocodiles when they dive into the water.
The breeding season varies according to latitude; spring (September-October) in southern Australia, April to July in northern Australia and winter (June-August) in southern Queensland.
European breeders winter in Africa. American and Canadian breeders winter in South America, although some stay in the southernmost U.S. states such as Florida and California. Australasian Ospreys tend not to migrate.
Studies of Swedish Ospreys showed that females tend to migrate to Africa earlier than the males. More stopovers are made during their autumn migration. The variation of timing and duration in autumn was more variable than in spring. Although migrating predominantly in the day, they sometimes fly in the dark hours particularly in crossings over water and cover on average 260-280 km/day with a maximum of 431 km/day.
Osprey populations declined drastically in many areas in the 1950s and 1960s; this appeared to be in part due to the toxic effects of insecticides such as DDT on reproduction. The pesticide interfered with the bird's calcium metabolism which resulted in thin-shelled, easily broken or infertile eggs. Possibly because of the banning of DDT in many countries in the early 1970s, together with reduced persecution, the Osprey, as well as other affected bird of prey species have made significant recoveries.