Like all birds, Ospreys have faced massive threats with the coming of industrialisation in the last two to three centuries, but even before that they were subject to persecution. There are records from the Middle Ages of Ospreys being killed when they were seen as a threat to stocks in fish ponds. But it was mankind's invention and use of the gun, the widespread use of pesticides and the "improvement" of natural habitats that caused the most substantial decline in numbers. In parts of their range, including England, shooting, habitat loss and the Victorian's obsession with collecting specimens and eggs caused the species to become extinct before the end of the 19th century.
Shooting, particularly of migrating birds in parts of Europe and of wintering birds in Africa, still takes its toll of Osprey populations. For example, click here to visit the site of the UK section of LIPU, the Italian League for Bird Protection - there is a very sad picture of a dead Osprey, one of many thousands killed for "sport", as it crossed from Sicily to Calabria in spring.
Ospreys became seriously threatened when pesticides such as DDT were widely used in the 1950s and 1960s. Residues of DDT accumulated in food chains, including fish - the major food of the Osprey. These residues concentrated in Ospreys at the top of the food chain, interfering with the female's ability to lay normal eggs. The eggshells became very thin and they were easily broken, dented, or crushed. It took many years before the effects of DDT on wild birds were properly understood, during which time breeding numbers of Ospreys fell by 50 to 90% in parts of its range in the United States.
In Scandanavia and Britain the effects were much less severe, perhaps because they tended to nest in areas where the pesticides were little used. Elsewhere in Europe, the effects of DDT may have been more marked. By the 1970s the use of DDT was phased out and since then breeding success of the Osprey has returned to pre-pesticide rates. In the 21st century the most significant threat to Osprey populations is threat to habitat. While in Europe the breeding sites are now strictly protected, it may well be that habitat loss on migration routes and at the wintering grounds provides a further threat for the future.