[order] Falconiformes | [family] Accipitridae | [latin] Pandion haliaetus | [UK] Osprey | [FR] Balbuzard pêcheur | [DE] Fischadler | [ES] Águila Pescadora | [IT] Falco pescatore | [NL] Visarend

Visarend determination

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The osprey is the only member of its family, the Pandionidae, and is a large bird of prey, 55-60 cm. in length and with a wingspan of 145-170 cm. Females are usually 5-10% larger than males, and weigh on average 1.6 kg as against 1.4 kg. for males. Both sexes have similar coloration, with the plumage being dark brown on the back, tail and upper wings. The head and body are white, with a dark eye stripe, a speckled crown and mottled light brown feathers on the upper breast. The flight feathers are grayish-brown and barred on the underside, and the primary feathers have long dark brown tips to them. The tail is similarly barred, and is darker brown at the tips. Other features include eyes with bright yellow irises, a black, sharply-curved beak and pale blue-grey feet. Vocalizations consist mainly of whistled notes that vary in pitch and intensity, depending on whether they are display calls or alarm calls etc.
The osprey has a number of specialized morphological adaptations for its fish-eating lifestyle, including nasal valves which close when it dives underwater. It has relatively long legs for a raptor and equal-lengthed toes, the outer of which is reversible, so that slippery fish can be grasped tightly with two toes in front and two behind. The talons are long and deeply-curved, and the undersides of the feet are covered in spiny bumps called spicules that help to hold the prey.

Ospreys are typically associated with permanent water habitats, especially sea coasts, impoundments, lakes, rivers, and swamps. Breeding habitat requirements include open expanses of water that support abundant slow-moving fish, water clarity sufficient to allow visual detection of fish, and elevated or inaccessible sites for nest-building. Osprey nest sites are highly variable, but most, if available, prefer either dead trees or living trees with broken or dead tops. The nest tree is usually taller than surrounding trees and as close to suitable foraging areas as possible. Where natural sites are limited or missing, Ospreys readily nest on human-made structures, such as power poles, radio towers, channel markers, television antennas, or bridges. The nest is a large, bulky structure that is regularly reused and enlarged for several successive years.

This species has a nearly world-wide distribution, being only absent from South America. Its European populations are wintering mainly in sub-Saharan Africa, but some birds remain in the Mediterranean region. Currently the total population of the European Union (12 Member States) can be estimated at 270-282 breeding pairs. It has undergone a dramatic decrease at the end of last century and beginning of this century. The species had been extirpated from several countries, including continental France, the British Isles, the Netherlands and Italy. Between 1930 and 1950 its population remained stable, and since 1970 the migratory populations of north-western Europe have increased. The species is consequently back in the British Isles and continental France. The Mediterranean populations, however, are still decreasing. Formerly direct persecution was the main threat to this species, but currently habitat destruction and pollution are more important

The diet consists almost entirely of fish, and the osprey is unusual amongst raptors in being so exclusively piscivorous. Both freshwater and marine fish are taken, and in the Highlands the main prey includes trout , pike and flounder. Hunting usually begins with a hovering flight over water, or occasionally from a perch overlooking water. When a fish is targeted, the osprey dives down with its wings swept back and just before impact it brings its talons forward to plunge feet-first into the water and grasp the fish under the surface.
Success rates in catching fish vary from about 20% to over 50% of attempts, depending on the ability of the individual osprey, and a bird can sometimes be completely submerged as it takes a fish. Flapping its wings strongly, the osprey rises out of the water and once airborne it carries the fish with one foot in front of the other, so that the head is facing forward - this is presumed to be a more aerodynamic position, which makes it easier for the osprey to fly with its prey. The fish is taken to a perch, often near the nest, where it is generally eaten headfirst.

This species has a large global range; the total size has not yet been quantified, but the Extent of Occurrence in Africa and the Americas combined is estimated to be 9,670,000 km². It has a large global population estimated to be 460,000 individuals (Rich et al. 2003). Global population trends have not been quantified, but the species is not believed to approach the thresholds for the population decline criterion of the IUCN Red List (i.e. declining more than 30% in ten years or three generations). For these reasons, the species is evaluated as Least Concern. [conservation status from birdlife.org]

Breeding begins when ospreys are three to five years old, and in Scotland, birds return at the end of March or early April. Courtship consists of aerial displays by the male bird near a nest site, which serve to attract potential mates and discourage rivals. Nests are known as eyries and are constructed out of sticks in the top of a tall tree, most often a Scots pine (Pinus sylvestris). Elsewhere in their range ospreys occasionally nest on cliffs. Ospreys pair for life and nests are used year after year, with large ones being up to a meter in width and height.
Mating takes place at the nest, and in late April or early May the female lays her clutch of eggs - usually three, but sometimes two or four. The eggs are a pale color with brown blotches, and are 62 x 46 mm. in size. Incubation is carried out by both birds, but only the male hunts at this time, sharing the fish with the female when she is sitting on the eggs. The chicks hatch after 35 days and the female broods them continuously, with the male bringing fish for her and the young. Unlike some other raptors, aggression amongst the chicks is rare and usually they all survive. As the chicks grow, their down is replaced by feathers, and they fledge at 53 days of age. For about two weeks they return to the nest to be fed by the male, while they strengthen their wings and learn to fish for themselves.

Migratory; mainly summer visitor to west Palearctic. Rare winter records known from north and central Europe. Less rare in winter around Mediterranean, and often seen then in North Africa, Iraq, and Persian Gulf, but main winter quarters of European and west Siberian birds in sub-Saharan Africa, where occur on river systems almost throughout, though evidently rare south of equator. Migrates on broad fronts over seas and deserts, and shows less inclination to concentrate at narrow sea-crossings than any other broad-winged raptor. Individuals often remain off-passage at suitable river, Lake, or reservoir sites for several days, or even weeks in cases of immature birds. Begin arriving African winter quarters late September to early October. Many young birds remain in wintering areas for their 1st summer or stop in Mediterranean basin; some 2nd-summer birds also stay south, and have given rise to persistent rumours of breeding in African interior. Adults start return towards breeding areas in March, mainly crossing Mediterranean second half March; early arrivals on breeding grounds late March, most during April. Returning 2nd-summer birds follow on average a month later than experienced breeders.