Osprey

Name and taxonomy

Carl Linnaeus described the osprey in 1758 and called it Falco haliaetus. Marie Jules Cesar Savigny introduced the genus Pandion in 1809. Pandion was a mythical king of Athens in Greek stories. The species name comes from the Greek words for sea and eagle. The English word osprey was first written down around 1460.

Four subspecies are usually recognised, and they differ only slightly. The Eurasian form ranges from the Azores east to Japan and Kamchatka. The North American form reaches from Alaska down into Central America. The Australasian form is the smallest, and some authors treat it as a species. Ridgway's osprey lives in the Caribbean and does not migrate.

Evolution and relatives

Molecular work places Pandionidae as the sister group of the hawks and eagles. The two families are estimated to have split about 50.8 million years ago. The osprey has always been awkward to classify. Some schemes fold it into the hawk family instead of its own.

Three extinct species of Pandion have been named from fossils. Pandion homalopteron came from Middle Miocene rocks in southern California. Pandion lovensis was described from Florida in 1985. The oldest fossils of the family come from Oligocene rocks in Egypt. Gerald Mayr described a claw from the Mainz basin in Germany in 2006.

Appearance

The upperparts are deep glossy brown and the underparts are white. A dark mask runs through the eye to the side of the neck. The bill is black, and the pale feet carry black talons. A black patch marks the wrist on the underside of each wing. In flight the wings arch and the tips droop, which looks gull-like.

Adults measure 50 to 66 cm long, with a span of 127 to 180 cm. Body weight runs from 0.9 to 2.1 kg. The wing chord measures 38 to 52 cm and the tail 16.5 to 24 cm. The Eurasian form averages 1.53 kg and the American form 1.7 kg. The Australasian form is lighter, at about 1.25 kg.

Toes of equal length set the osprey apart from other day-flying raptors. The talons are rounded in cross section rather than grooved. Young birds show buff edges on the back feathers, and their eyes are orange. Adult eyes are yellow, and the plumage change finishes by 18 months. The legs are pale grey, not the yellow of most other raptors.

Habitat and range

The osprey is the second most widespread raptor, after the peregrine falcon. It is one of only six land birds found nearly worldwide. It breeds beside lakes, rivers, estuaries and sheltered coasts. Nests usually sit within 3 to 5 kilometres of water. In South America the bird turns up only as a non-breeding visitor.

European birds breed north into Scandinavia, Finland and Britain. North American birds breed from Alaska south to the Gulf Coast. Australian birds sit patchily around the coast and mostly stay put. A gap of 1,000 kilometres along the Nullarbor coast splits that range. Fossil bones from Tonga suggest arriving people wiped the bird out there.

A safe nest site matters more to the osprey than any one habitat. It uses dead trees, cliffs, islets, buoys, poles and built platforms. Beaver dams create the shallow ponds and standing dead trees it likes. Nests placed over water stay safest from climbing predators.

Diet and hunting

Fish make up 99 per cent of the prey. Typical catches weigh 150 to 300 grams and run 25 to 35 centimetres long. The bird can handle anything from 50 grams up to 2 kg. Northern pike of 2.8 kg have been taken in Russia. Studies in North America list more than 80 fish species in the diet.

The osprey searches on the wing, 10 to 40 metres above the water. It hovers for a moment, swings its legs forward and plunges. It often submerges completely, then lifts off with heavy wingbeats. It corrects for the way water bends the image of the fish. Between 24 and 74 per cent of dives end in a catch.

Once airborne the bird turns the fish to face forward. That grip cuts drag on the flight back to a perch. The osprey eats from the head towards the tail. It does not store fish, and it rarely takes carrion. Fish flesh holds enough water, so the bird seldom drinks.

Other prey turns up now and then. Records include voles, squirrels, muskrats, snakes, birds and a small alligator. Dead white-tailed deer and opossums have both been eaten.

Behaviour and social life

Ospreys defend the nest but not a hunting territory. Fish move about, so guarding a feeding area would not pay. Nests can sit many kilometres apart or less than 100 metres apart. Loose colonies form where good sites cluster together. Birds sometimes fish in groups, which may raise the success rate.

A breeding bird may travel up to 14 kilometres from the nest to fish. Non-breeding birds move up to 10 kilometres between roost and feeding ground. Pairs return to the same nest year after year. They repair it each season and add fresh sticks before laying. Some Australian nest sites have been in use for 70 years.

Migration splits the species roughly along a line of latitude. European breeders winter in Africa, and many cross the Sahara twice a year. American breeders reach South America, though some stop in Florida. Migrating birds cover 260 to 280 kilometres a day on average. The best day recorded was 431 kilometres.

Communication and senses

Its voice is a run of sharp whistles. Researchers recognise up to five separate calls. Each call usually goes with a posture or a flight pattern. A bird disturbed at the nest gives a frantic alarm note.

The sky-dance is the best known display. A male carries a fish or a stick and screams while he flies. He alternates short rising flights with spells of hovering. The display runs through courtship and into early incubation. A female begs from her mate, and sometimes from a neighbouring male.

Vision is tuned to spotting objects below the surface from the air. A pale nictitating membrane covers the eye during a dive. The nostrils close to keep water out. Dense oily plumage stops the feathers from waterlogging.

Reproduction and life cycle

Ospreys usually mate for life. Migrating males often reach the nest several days before the female. Both birds gather material, and the female arranges most of it. The nest is a heap of sticks, driftwood, turf or seaweed. A big nest on a pole can be 2 metres wide and weigh about 135 kg.

The female lays two to four eggs, a day or two apart. Each egg is about 6.2 by 4.5 centimetres and weighs around 65 grams. Incubation lasts about 35 to 43 days. The eggs hatch in the order they were laid. The first chick out is bigger and beats the others to food.

A newly hatched chick weighs only 50 to 60 grams. The male brings 60 to 100 grams of fish an hour through the day. That works out at 3 to 10 fish delivered to the nest daily. The female broods almost without a break for the first two weeks. Chicks reach 70 to 80 per cent of adult size within a month.

Young birds fledge between 48 and 76 days old. The parents keep feeding them for two to eight weeks after that. Most ospreys first breed at about three years old. Where nest sites are scarce, first breeding slips to five years or later. Typical lifespan is 7 to 10 years, and a few birds pass 25.

Predators and threats

Bald eagles and great horned owls take osprey chicks and sometimes adults. Raccoons and snakes are suspected raiders of eggs and nestlings. That risk explains why so many nests sit over open water. The speckled down of a chick probably hides it from birds overhead.

Nile crocodiles kill ospreys that bathe or roost by African water. In Florida, introduced pythons and anacondas can take the birds. Migration is the dangerous season for a Swedish osprey. More than half of the yearly deaths happen on the move. Crossing the Sahara in spring kills many of them.

Power cables and aircraft cause further losses. A nest on a utility pole can start a fire and cut the electricity supply. Egg collectors and shooting hit the species hard around 1900.

Population and conservation

The IUCN Red List rates the osprey as Least Concern. The global population is estimated at 460,000 birds. The range covers 9,670,000 square kilometres in Africa and the Americas alone. No worldwide decline of the size the Red List needs has been measured.

Numbers fell sharply in the 1950s and 1960s. DDT upset calcium chemistry and left shells thin or eggs infertile. Many countries banned the pesticide in the early 1970s. Populations recovered as the poison faded and persecution eased. Artificial nest platforms helped as well.

Farmers found the platforms useful, because ospreys drive off red-tailed hawks. Regional declines continue in South Australia. Coastal recreation and building press on nest sites there. Old territories on the Spencer Gulf have stood empty for decades.

The bird died out as a British breeder by the early 20th century. Two Scandinavian ospreys nested at Loch Garten in 1954. The charity later bought the reserve, and it has guarded the site since then. Roughly 400 pairs breed in the United Kingdom today.

Relationship with humans

People watch ospreys far more than they trouble them today. Nest cameras and viewing hides draw visitors to breeding sites. Loch Garten in Scotland held a pair every year from 1959 to 2019. Money from those visitors supports rural areas near the lochs.

Ospreys tolerate short visits by researchers at the nest. The nests are conspicuous, which makes the birds easy to study. Plans for nesting platforms are published free and used worldwide. One design became the official pattern for the state of New Jersey. A global site maps nest locations and logs breeding success.

Ospreys share their nests with other birds. Grackles, swallows, starlings and sparrows build inside the stick pile. Geese, herons, eagles and owls sometimes take a nest over in spring. Plumes sold as osprey in the hat trade actually came from egrets.

In culture and myth

Pliny the Elder wrote that osprey parents made their young fly at the sun. He claimed the birds killed any chick that failed the test. Gerald of Wales passed on a story that the bird had one webbed foot. A medieval belief held that fish turned belly up before an osprey. Shakespeare used that belief in Coriolanus.

The opening poem of the Chinese Classic of Poetry begins with ospreys calling. Guan ju has been dated to the seventh century BC. In it the birds stand for fidelity between husband and wife. The Analects records that Confucius praised its balance of joy and sorrow. Some commentators argue the bird in the poem is a duck instead.

The Irish poet W. B. Yeats gave a grey wandering osprey the role of sorrow. In heraldry the bird appears as a white eagle holding a fish. There it stands for vision and abundance. More than 50 postage stamps carry the species.

Nova Scotia named the osprey its provincial bird in 1994. Oregon made it the state raptor after a debate in 2017. Sports teams take the name, including a rugby side based in Swansea. The Seattle Seahawks won their name from a contest with 1,740 other entries.