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In 1971, in collaboration with the Museum of Natural History we started a census of Osprey. The origin of this collaboration was the poor breeding performance of Ospreys seen at the end of the 1960s. Almost simultaneously, reports came from abroad about the thinning of their egg shells that affected many bird species and could be attributed to the use of DDT. It turned out that Swedish Ospreys were influenced by this; at its worst around 15% of the population (in the early 1970's). The ospreys, according to our studies were not back to their normal shell thickness again until early in the 21st century, therefore it took 30 years to recover. Until 2003, it was an upward trend for the osprey population, but 2008 showed that some populations declined, possibly due to increased competition from the White-tailed eagle.