Most resident ospreys lay their clutch in winter. The area around the nest is generally open, giving the birds clear access when landing. Ospreys also prefer to nest over water for protection against climbing predators. Islands free of mammalian predators allow safe nesting in low trees and even on the ground. Therefore, they generally catch only surface fish or those that frequent shallow flats and shorelines.
Ospreys are opportunists.
Predators exert a major impact on the nest sites ospreys choose. In the 1960's osprey populations declined as a result of DDT which were washed into water courses and ingested by fish. Following tight restrictions on the use of DDT, pesticide residues declined, and North American osprey populations increased consistently between 1968 and 1981. Some bird species have been observed forming protective nesting associations with ospreys by building their nests in the sides or bottoms of the stick nests of ospreys. Artificial nest sites are successfully used by ospreys.