Among North America’s best-studied birds of prey, the widely admired Osprey is the continent’s only raptor that plunge-dives to catch live fish as its main prey source. Despite its reliance on fish, Ospreys occupy a broad array of habitats, ranging from mangrove islets of the Florida Keys to Alaskan lakes, and from New England salt marshes to the saline lagoons of Baja California. Northern populations migrate south to overwinter on fish-rich rivers, lakes, and coastal areas of Central and South America, returning north each spring as waters warm and fish become accessible. An Osprey nesting in central Québec and overwintering in southern Brazil might fly more than 200,000 kilometers during its 15- to 20-year lifetime.
Ospreys dive feet first to capture their prey, accessing only about the top meter of water, so they are restricted to foraging for surface-schooling fish and to those in shallow water—the latter generally are most abundant and available. As a result, North America’s Ospreys tend to breed most densely where shallow waters abound: Long Island Sound, Chesapeake Bay, and Florida Bay along the Atlantic coast; Georgian Bay in the Great Lakes; the Pacific coast of Baja California; and large reservoirs and lakes in western states. In many of these places, artificial nest sites, or nest platforms, have helped breeders enormously in recent decades. Historically, Ospreys built their bulky stick nests atop trees, rocky cliffs, promontories, and even on the ground on a few islands that lack mammalian predators. While some continue to use natural sites, many have shifted to nesting on artificial structures. Ospreys now use an astonishing array of artificial sites: channel markers in harbors and busy waterways; towers for radio, cell phone, and utility lines; and platforms erected exclusively for the species. This shift has been dramatic in many regions, with 90–95% of pairs choosing to nest at artificial sites; predation, loss of trees, and development of shorelines have been driving forces behind the change.
North American Ospreys gained increased recognition from the 1950s to 1970s because populations crashed in several key regions. About 90% of the pairs nesting along the Atlantic coast between New York City and Boston, for example, disappeared during this period; Chesapeake Bay lost more than half of its breeding pairs, and Great Lakes populations also suffered major declines. Studies showed that high levels of contaminants (especially DDT and its derivatives) in eggs, severe eggshell thinning, and poor hatching success were responsible. Mortality of adults may have contributed to the decline. Osprey studies provided key evidence in court to block continued use of persistent pesticides, and Osprey populations recovered rapidly thereafter. Although small pockets of contamination remain, the historic range has greatly expanded and many U.S. and Canadian populations now exceed historical numbers, owing to a cleaner environment, increasingly available artificial nest sites, and this bird’s ability to tolerate human activity near its nests. Phoenix-like, the Osprey has arisen from the ashes of its own demise, a survivor, even a backyard bird in some areas today. Indeed, there is little wonder the species has become such a powerful totem for conservation.
Concern for populations impacted by pesticides spawned a multitude of studies on Osprey life history during the 1970s and 1980s, and these have continued almost undiminished since then—the product of Osprey allure, broad distribution, and ease of study (highly visible nesting and hunting, and accessible nests). The life history of the North American Osprey was summarized by Bent 1937b, Palmer 1988e, and Poole 1989a, while Cramp and Simmons 1980b and Dennis 2008 did the same for Osprey breeding in the Palearctic. Although this account, by necessity, leans on these earlier works, we have emphasized more recent studies. Areas of important new research include: analyses of migratory movements based on satellite tracking (Martell et al. 2001a, Martell et al. 2014, Washburn et al. 2014); navigational abilities (Horton et al. 2014); behavior at the nest (Birkhead and Lessels 1988, Bretagnolle and Thibault 1993); molecular genetic studies to determine Osprey systematics (Seibold and Helbig 1995b) and phylogeography (Monti et al. 2015); foraging and development of dietary preferences in the postfledging period (Edwards and Collopy 1988, Edwards 1989b); assessment of contaminants in western populations (Henny et al. 1991b, Elliott et al. 1994, Elliott et al. 2000, Elliott et al. 2001b); status of Great Lakes populations (Ewins 1995, Ewins 1996, Ewins 1997, Ewins et al. 1995a, Ewins et al. 1999); sibling aggression among nestlings (Forbes 1991); colonial nesting (Hagan and Walters 1990); lifetime reproduction (Postupalsky 1989b); and growth of nestlings (Steidl and Griffin 1991, Schaadt and Bird 1993).