One Pair of Breeding Osprey in Contra Costa County

Erecting artificial nest platforms could help the Osprey

by James Hale

The Osprey (Pandion haliaetus) is a fish-eating, diurnal bird of prey with a cosmopolitan range. Sometimes called fish eagle, sea hawk, river hawk, or fish hawk, this large raptor is found in a wide variety of habitats on all continents except Antarctica. It nests near large bodies of water with an adequate supply of fish. The fossil record identifies two extinct species of Osprey. A number of claw fossils have been identified from California, Florida, South Carolina, and Egypt. Four subspecies are recognized, with Pandion haliaetus cristatus recently given full species status. The genus name of Pandion refers to the mythical Greek king Pandion of Athens who was transformed into an eagle. The specific epithet haliaetus is from the Greek for “fishery eagle”. The word Osprey was first recorded in 1460 and derived from Anglo-French and Medieval Latin. The Osprey is depicted in many cultures worldwide. Several characteristics separate the Osprey from other diurnal birds of prey. Its talons are rounded rather than grooved, its tarsi are reticulate, and its toes are of equal length.

Ospreys are dark brown above and white below. The all white head has a prominent dark eye stripe. Females have a dark breast band that is weaker or non-existent in the male. Adult Ospreys may reach five pounds in weight, twenty-six inches in length, and with a seventy-two inch wingspan. The arched wings with dark “wrists” are characteristic in flight. The call is a series of sharp whistles, often heard while foraging or near the nest. After the Peregrine Falcon, the Osprey is the second most widely distributed raptor species. In Contra Costa County, the Osprey is an uncommon migrant and wintering bird. Most sightings come from the various watershed reservoirs and the Richmond shoreline from August through April.

Ninety-nine percent of the Osprey’s diet is comprised of fish. The prey is first sighted when the Osprey forages over water at up to one hundred and fifty feet above. After hovering briefly, the Osprey plunges feet first into the water, up to three feet deep, with fish of various species taken up to five pounds. The reversible outer toes, closable nostrils, and “barbed” talons have adapted the Osprey to its fish diet. After capture, the fish is oriented so that it faces forward and carried to a perch, nest, or snag where it is consumed. On occasion, rodents, rabbits, hares, reptiles, amphibians, and other birds are taken as prey.

Ospreys breed near fresh water lakes, rivers, and coastal brackish waters after reaching sexual maturity at three to seven years of age, depending upon population densities. Their elaborate courtship displays include circling together while calling, with undulating flights by the male. Nest sites are often located on tree snags, high-tension towers, rocky cliffs, or other man-made structures. Nests are constructed of sticks and may be quite large. Ospreys are known for using odd bits of building materials such as old plastic tarps, tattered flagging, and Styrofoam cups for nest construction. Ospreys usually mate for life with nesting occurring from April through August. Two to four white eggs with bold reddish-brown splotches are laid within a month and incubated for about five weeks to hatching. The nestlings fledge in eight to ten weeks. The typical lifespan is seven to ten years, although rarely individuals may grow to twenty-five years of age. The oldest European Osprey recorded in the wild lived to be over thirty years of age. Other than humans, Great Horned Owls, Bald Eagles, Golden Eagles, and raccoons are the Ospreys major predators.

Ospreys have a large range covering more than 4,250,000 square miles globally, with a worldwide population estimated at over 500,000 individuals. Hunting of adults and egg collectors were the main threats to Osprey populations in the late 19th and 20th centuries. The toxic effects of insecticides such as DDT and DDE, causing eggshell thinning and reproduction failure, led to population declines in the 1950’s and 1960’s. Most North American populations have recovered after banning these insecticides. The Osprey is a California Species of Special Concern. There is one known breeding pair in Contra Costa County near Point Pinole Regional Shoreline. It is possible that the more widespread erection of artificial nest platforms may lead to further recovery of nesting Ospreys and local populations in general.

James M. Hale is a wildlife biologist, ethnobiologist and ecological consultant based in Contra Costa County. Visit his website at www.dochale.com

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