Osprey

• 1. Introduction

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  The osprey, Pandion haliaetus, eats almost nothing except fish. Fish account for 99 percent of its diet. Older names capture the habit: sea hawk, river hawk and fish hawk. This large daytime bird of prey lives on every continent except Antarctica. In South America it appears only as a migrant outside the breeding season. Several puzzles run through its story. How does a bird spot a fish from the air and then catch it underwater? Why have scientists struggled to decide which bird family it belongs to? What sent its numbers sharply downward, and what helped it recover? And why did a state legislature end up in a spirited argument over it?

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• 2. The Dive

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  Somewhere between 10 and 40 metres above the water, an osprey first sights its prey. It hovers for a moment, then plunges feet first into the water. Often the whole body goes under. Its eyes are well suited to picking out underwater objects from above. Water bends light, so a fish appears slightly displaced from where it truly swims. During the dive, the bird adjusts its flight angle to correct for this refraction.

  The usual catch is a live fish of 150 to 300 grams and about 25 to 35 centimetres long. Almost any fish from 50 grams up to 2 kilograms is fair game. In Russia, ospreys have taken northern pike weighing 2.8 kilograms. Dead or dying fish are rarely taken. After a successful dive, the bird usually eats on a nearby perch, though some carry their fish a longer distance.

  Occasionally the menu stretches to rodents, rabbits, hares, other mammals, snakes, turtles, frogs, birds, salamanders, conchs and crustaceans. Carrion is seldom reported. Even so, ospreys have been seen eating a dead white-tailed deer and Virginia opossums. Holding on to a slippery fish takes equipment that almost no other raptor carries.

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• 3. Built for Fish

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  Only two kinds of raptor can swing an outer toe backward, and the osprey is one of them. The other is the owl. This lets the osprey grip prey with two toes in front and two behind, a real help with slippery fish. Its toes are all the same length, its lower legs are covered in a net-like pattern of scales, and its talons are rounded rather than grooved. Sharp spicules cover the underside of the toes, and backward-facing scales on the talons work like barbs. The nostrils close against water during a dive. Dense, oily plumage keeps the feathers from soaking through.

  An adult weighs between 0.9 and 2.1 kilograms and measures 50 to 66 centimetres in length. The wingspan runs from 127 to 180 centimetres, similar to the largest buteo hawks and falcons. The upperparts are a deep, glossy brown, and the underparts are pure white. The breast is white, sometimes streaked with brown. A white head carries a dark mask across the eyes that reaches the sides of the neck. The irises range from golden to brown, and the transparent third eyelid is pale blue. The bill is black with a blue cere, and the feet are white with black talons. Black wrists on the underside of the wings serve as a field mark.

  Long, narrow wings with four finger-like feathers and a shorter fifth, plus a short tail, give the bird a distinctive outline. In flight the wings arch and the hands droop, which makes it look like a gull. The male is slimmer than the female, with narrower wings and a weaker breast band, or none at all. Telling the sexes apart is easy in a breeding pair but harder with a lone bird. A young osprey shows buff fringes on the upperpart feathers, a buff tone below and streaked feathers on the head. In spring, barring on the underwings and flight feathers is a better clue, because the upperparts have worn.

  The call is a series of sharp whistles, rendered as cheep, cheep or yewk, yewk. Disturb a nest, and the whistle turns into a frenzied cheereek.

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• 4. Names and Classification

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  In 1758, Carl Linnaeus gave the osprey its first scientific name, Falco haliaetus, in the 10th edition of his Systema Naturae. He gave the type locality as Europe. In 1761 he narrowed it to Sweden. The genus Pandion followed in 1809, introduced by the French zoologist Marie Jules César Savigny. Today the osprey is the only living species in that genus, and the genus is the only member of the family Pandionidae.

  Pandion is the name of a mythical Greek king of Athens, the grandfather of Theseus. The species name haliaetus comes from the Greek haliaetos, meaning sea-eagle, built from hals for sea and aetos for eagle. The English word osprey is first recorded around 1460. It came by way of the Anglo-French ospriet and the Medieval Latin avis prede, meaning bird of prey. The Oxford English Dictionary notes a possible link to ossifraga, the Latin word for bone breaker used by Pliny the Elder. Pliny, however, was describing the bearded vulture.

  Where the family belongs has always been a riddle. The traditional placement puts it in the order Accipitriformes. Other schemes tuck it beside the hawks and eagles in the family Accipitridae. The Sibley-Ahlquist taxonomy grouped it with the other daytime raptors in a greatly enlarged Ciconiiformes. That grouping is paraphyletic, meaning it leaves out some descendants of a shared ancestor, so it is considered unnatural. Molecular analysis shows that Pandionidae is the sister family of Accipitridae.

  The fossil record names three extinct species. In 1976, Stuart L. Warter described Pandion homalopteron from marine Middle Miocene deposits of the Barstovian age in southern California. Pandion lovensis, described in 1985 from Florida, dates to the Late Clarendonian and may represent a separate lineage. Claw fossils from Pliocene and Pleistocene sediments have turned up in Florida and South Carolina.

  The oldest recognized fossils of the family come from the Oligocene Jebel Qatrani Formation in Faiyum Governorate, Egypt. They are too incomplete to assign to a genus. Another Oligocene species, found in Hungary, was named P. pannonicus. In 2006, Gerald Mayr described one more claw, from Early Oligocene deposits in the Mainz basin of Germany.

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• 5. Across Continents

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  Only six land birds have a worldwide distribution, and the osprey is one of them. Among raptors, only the peregrine falcon is more widely distributed. Despite that reach, the osprey splits into just a few subspecies, and even those are not cleanly separable. Four are generally recognised, though ITIS lists only the first three below.

  The American osprey, P. h. carolinensis, ranges from Alaska and Canada through much of Central and South America, except Chile and Patagonia. It is larger than its European cousin, averaging 1.7 kilograms, with a darker body and paler breast. The Australasian osprey, P. h. cristatus, is the smallest and most distinctive, averaging 1.25 kilograms. It lives along the entire marine coastline of Australia, on some larger freshwater rivers, and in Tasmania. Some authorities treat it as a full species, Pandion cristatus, also called the eastern osprey. A 2018 genetic study using microsatellite data found only low divergence between it and the other subspecies.

  The Eurasian osprey, P. h. haliaetus, is the nominate subspecies and averages 1.53 kilograms. It spans the Palearctic and parts of sub-Saharan Africa, from the Azores and the Iberian Peninsula east to Japan and the Kamchatka Peninsula. It also reaches South and Southeast Asia, Madagascar and much of the African coastline. Ridgway's osprey, P. h. ridgwayi, lives in the Caribbean islands. It has a very pale head and breast and only a weak eye mask, and its name honours Robert Ridgway.

  In North America, ospreys breed from Alaska and Newfoundland south to the Gulf Coast and Florida. In Europe they spend summer as far north as Ireland, Scandinavia, Finland and Great Britain, but not Iceland. Australian birds are mostly sedentary and found patchily around the coast. A gap of 1000 kilometres, matching the coast of the Nullarbor Plain, separates the westernmost breeding site in South Australia from the nearest ones in Western Australia. In the Pacific, ospreys are found in the Bismarck Islands, the Solomon Islands and New Caledonia. Fossils of adults and juveniles in Tonga suggest they were probably wiped out there by arriving humans. They may once have ranged over Vanuatu and Fiji as well.

  Most populations move with the seasons. European breeders winter in Africa, and American and Canadian breeders winter in South America. Some stay in the southernmost American states such as Florida and California. In the Mediterranean, some birds stay put while others make short trips. An osprey tagged in Norway was tracked to western India, so European birds may sometimes winter in South Asia.

  Studies of Swedish ospreys found that females head for Africa earlier than males. Autumn trips include more stopovers and vary more in timing than spring trips. The birds travel mostly by day but sometimes fly in the dark, especially over water. They cover an average of 260 to 280 kilometres a day, with a maximum of 431.

  Migration is also when ospreys die. More than half of the annual deaths among Swedish ospreys occur during the migration seasons. In spring, the toll falls mainly in Africa and is traced to the crossing of the Sahara. In Florida, invasive snakes including Burmese pythons and green anacondas may eat ospreys at some stage of their growth. Human infrastructure adds its own hazards, from nests beside overhead electric cables to collisions with aircraft.

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• 6. Nest and Family

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  On Rottnest Island, off the coast of Western Australia, ospreys nest on rocky outcrops just offshore. About 14 such sites are known, and five to seven are used in any one year. Many are renovated every season, and some have been in use for 70 years.

  Elsewhere, the nest is a large heap of sticks, driftwood, turf or seaweed. It sits in the fork of a tree, on a rocky outcrop, on a utility pole, on an artificial platform or on an offshore islet. Nests can reach 2 metres across and weigh about 135 kilograms. Large ones on utility poles can be fire hazards and have caused power outages.

  Ospreys usually begin breeding at three to four years old. In places with many ospreys, such as Chesapeake Bay in the United States, they may wait until five to seven. A shortage of tall structures can force young birds to delay. People erect posts to help. A nesting platform design from Citizens United to Protect the Maurice River and its Tributaries, Inc. became the official design of the State of New Jersey. Its plans and materials list are available online and have been used by people in many regions. A global site maps osprey nests and logs how well they succeed.

  The female lays two to four eggs within a month and relies on the nest's bulk to hold in heat. The eggs are whitish with bold reddish-brown splotches, about 6.2 by 4.5 centimetres and about 65 grams. Incubation lasts about 35 to 43 days. Newly hatched chicks weigh only 50 to 60 grams, yet they fledge within 8 to 10 weeks. A study on Kangaroo Island, South Australia, found an average of 69 days between hatching and fledging.

  The same study counted 0.66 young fledged per year per occupied territory and 0.92 per active nest. About 22 percent of surviving young stayed on the island or came back at maturity to breed. When food runs short, the first chicks to hatch are the most likely to survive. A typical osprey lives 7 to 10 years, and a few reach 20 to 25. The oldest wild European osprey on record reached 26 years and 11 months.

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• 7. Poison and Recovery

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  An estimated 460,000 ospreys live in the world today. In Africa and the Americas alone, their range covers 9,670,000 square kilometres. Global trends have not been measured. Even so, the species does not seem close to the IUCN Red List threshold of a decline of more than 30 percent in ten years or three generations. It is rated least concern.

  The picture was grimmer in the late 19th and early 20th centuries, when egg collectors and hunters of adult birds were the main threats. Osprey populations then fell drastically in many areas during the 1950s and 1960s. The cause appeared to be partly the insecticide DDT, which disrupted the birds' calcium metabolism. The result was thin-shelled eggs that broke easily or were infertile. Ospreys and other affected birds of prey have since made significant recoveries, possibly because many countries banned DDT in the early 1970s and persecution eased.

  Artificial nesting sites helped as well. Farmers found that building them offset habitat loss, and the ospreys that moved in drove off red-tailed hawks that preyed on chickens.

  South Australia tells a different story. Former territories in the Spencer Gulf and along the lower Murray River have stood vacant for decades. Nesting sites on the Eyre Peninsula and Kangaroo Island are vulnerable to unmanaged coastal recreation and encroaching urban development.

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• 8. Legends and Symbols

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  The Roman writer Pliny the Elder reported that parent ospreys forced their young to fly up toward the sun as a test, and killed any that failed. A stranger legend came from Gerald of Wales, then Albertus Magnus, and was recorded in Holinshed's Chronicles. It held that the bird had one webbed foot and one taloned foot. A medieval belief said fish were so mesmerised by the osprey that they rolled belly-up in surrender. Shakespeare refers to it in Act 4, Scene 5 of Coriolanus.

  In China, the folk poem "guan guan ju jiu" features the osprey as ju jiu, with guan guan imitating its voice. Because ospreys pair so faithfully, the poem treats the bird as an emblem of fidelity and harmony between husband and wife. Some commentators argue that the bird is really a mallard duck, since the osprey cannot make that sound. In 1889, the Irish poet William Butler Yeats used a grey, wandering osprey to stand for sorrow in The Wanderings of Oisin and Other Poems.

  Heraldry usually draws the osprey as a white eagle, often with a fish in its talons or beak, and calls it a sea-eagle. It has historically stood for vision and abundance. More recently it has become a symbol of positive responses to nature and has appeared on more than 50 international postage stamps. In 1994, Nova Scotia named it the provincial bird.

  Sports teams borrow the bird as well. The North Florida Ospreys of the University of North Florida and the Missoula Osprey baseball team are two examples. Seahawks, another old name for the bird, is more common still. The Seattle Seahawks of the National Football League took their name from a naming contest and beat 1,740 other entries. General manager John Thompson said the name "shows aggressiveness, reflects our soaring Northwest heritage, and belongs to no other major league team."

  In the late 19th century, so-called osprey plumes were prized in the plume trade and decorated hats, including those worn with army uniforms. Despite the name, they came from egrets. In Oregon, the 2017 legislative session brought a short-lived dispute over whether the western meadowlark or the osprey should be the state bird. State Representative Rich Vial played the meadowlark's song on his smart phone over the House microphone. The compromise, SCR 18, passed on the last day of the session. It named the western meadowlark the state songbird and the osprey the state raptor.

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