Osprey (Pandion haliaetus)

Legal status in WI: threatened 1991 numbers in WI: 340 active territories Length: 23 inches Wingspan: 5 feet Weight: 3-3.5 pounds

Description

The osprey is our only bird of prey that plunges into the water to capture fish. Ospreys are large birds, black-brown on top and white below. Sometimes people confuse them with bald eagles. The osprey’s white head has a broad, black cheek patch and its tail is barred with brown, whereas a mature eagle’s head and tail are entirely white. The osprey’s long, narrow wings are angled back at the wrist when it flies, and a black wrist patch contrasts with white underwings. Bald eagles soar on broad, straight, dark wings. Ospreys also are smaller than eagles.

Male and female ospreys are similar in appearance, although females may have a necklace of dark streaking. An osprey’s voice is a series of sharp whistles and chirps, which become more frenzied if you approach the nest.

Food

Wisconsin’s ospreys feed primarily on fresh panfish. They search for fish while flying 30-100 feet above the water. If they spot prey, they hover, then plummet down, plunging into the water feet first. Strong, curved talons and specialized spines on the bottoms of their feet help them hold onto the slippery fish. If they catch the fish, they quickly take off, juggling it in their feet until its head is facing forward into the wind. Then they fly to a perch to eat it. Ospreys can’t swim and have been known to drown, especially if they get their talons stuck in too large a fish and can’t take off.

Breeding Biology

Ospreys are summer residents in Wisconsin. They return to the state from southerly wintering grounds in mid-April. When 3-4 years old, ospreys are sexually mature and will mate and nest. The birds are territorial, defending their nest area from other ospreys. They select nesting sites near water and build atop dead or living trees, or on power poles and other human made structures. Sometimes they use old bald eagle or great blue heron nests. Mated pairs may repair and use the same nest year after year.

The large nest, usually more than 3 feet in diameter, is made of big sticks and lined with grasses and pond weeds. The female lays an average of 3 buff-colored, brown-speckled eggs, which she incubates for about 28 days. The male brings fish to the female, who leaves the nest only occasionally to exercise her wings.

Osprey chicks are especially vulnerable to the weather because their brownish natal down doesn’t thicken or change into a warmer, second natal coat, as it does in other birds or prey. Thus, the female remains at the nest to shield them from sun, rain and cold. At this time, people should stay away from osprey nests to avoid frightening the female away. When danger is near, the female signals to the young, who "play dead." Predators like great horned owls, raccoons, ravens and crows will eat osprey chicks.

For the first few weeks, the female feeds the chicks semi-digested, regurgitated food. Both parents feed older chicks small bits of fish. Young ospreys fledge when about 8 weeks old, usually by mid-August. They stay near the nest and their parents until October. Then ospreys begin their migration south.

Due to exposure, predation and accident, ospreys have a high death rate during their first 2 years. An average of 1 chick per nest fledges each year. Once the birds reach adulthood, however, their chances for long-term survival are good. Ospreys can live as long as 20 years.

Distribution

Ospreys occur on all continents except Antarctica. Because they eat fish, they live along rivers, lakes and coastlines.

In Wisconsin, ospreys breed mainly in the lake regions of northwest and north central Wisconsin. A few nest along the Wisconsin River in the central part of the state. Wisconsin’s ospreys overwinter from the southern U.S. coasts to Central and South America. A map outlining Pre-1977 and 1997 to Present Distribution is available.

History in Wisconsin

Ospreys never were numerous in Wisconsin, but they nested in suitable habitat throughout the state. Populations in Wisconsin and across the U.S. began to decline in the 1950s, largely due to the adverse impacts on reproduction of organochlorine pesticides like DDT. These pollutants caused females to lay eggs with thin shells that broke under the weight of an incubating adult. Few chicks survived, and populations plummeted.

In 1972, use of organochlorine pesticides was banned in the U.S., resulting in a gradual increase in osprey populations. Residues still remain in the environment, however, and these pesticides are still manufactured in the U.S. and sold for use in other countries. Many countries in Central and South America, where ospreys overwinter, still use DDT. Thus, some ospreys continue to be exposed to the pesticides and their adverse effects.

Nest blow-downs, habitat destruction and disturbance (e.g., waterfront development, boating, shooting) and predation also continue to influence the reproductive success of ospreys in Wisconsin.

Current Status

Ospreys were placed on the Wisconsin Endangered and Threatened Species List in 1972. Since the banning of DDT, populations have recovered to a present level of more than 300 nesting pairs in the state. In 1989 the osprey was upgraded to Threatened status. By remaining on the Endangered and Threatened Species List, ospreys will still receive protection.

Research and Management

Since 1973, the DNR has conducted annual aerial surveys to locate active osprey nests.

Biologists then visit each nest to band the chicks and observe fledgling success. Injured or sick chicks are removed for rehabilitation, and dead chicks and eggs are collected and analyzed for contaminants.

Research in the mid-1970s showed that 10% of all osprey nests blew down each year. Thus, DNR biologists began constructing artificial nesting platforms which are now used by 2/3 of Wisconsin’s ospreys. On average, osprey nesting success is now better because platforms provide a more stable site.

Once the DNR knows a nest’s location, it develops a management plan for the area using guidelines developed by the U.S. Forest Service for federal lands. On both federal and state lands, timber harvest, road building and other human disturbances are restricted within 1/4 mile of an osprey nest. The DNR enlists the cooperation of landowners to protect osprey nests on private lands.

The DNR’s Bureau of Endangered Resources (BER) is enacting an Osprey Recovery Plan. The goal, to increase the number of breeding pairs in the state to 300, was reached in 1990. The BER plans to continue to survey the osprey population, protect ospreys and their habitat, and establish communication and education programs to keep citizens informed about osprey recovery work.

What You Can Do

You can assist the BER in its important work by: reporting osprey nests to the BER at the address below; avoiding osprey nests during breeding season; working to preserve osprey habitat; reducing your use of pesticides; supporting legislation banning harmful pesticides; and contributing to the Endangered Resources Fund on your Wisconsin income tax form.

Further Reading

Bent, A.C. 1937. Life Histories of North American Birds of Prey, Part I. Bull. 167, U.S. Natl. Museum, Washington, D.C. 409 pp.

Eckstein, R.G., P.V. Vanderschaegen and F.L. Johnson. 1979. Osprey nesting platforms in north central Wisconsin. Passenger Pigeon. 41:145-148.

Sindelar, C., Jr. 1971 Wisconsin osprey survey. Passenger Pigeon. 33:79-88

Excerpt from THE ENDANGERED AND THREATENED VERTEBRATE SPECIES OF WISCONSIN

Status: State Threatened (1989).

Occurrence: Uncommon migrant and summer resident north; rare summer resident central. The osprey breeding population is concentrated largely in the inland lake region of northern Wisconsin. Small populations are nesting in central Wisconsin. A map outlining Pre-1977 and 1997 to Present Distribution is available.

Aid to ID: A large raptor (bird of prey), black-brown on top and white below. Diagnostic characteristics include a crook in the long wings which have a predominant black marking on the undersides.

Habitat: Habitat includes forested areas near the Great Lakes, inland lakes and swamps. Areas are usually forested with second growth pine, aspen, and hardwood forests. Most frequent nest sites are supercanopy snags and dead-topped pines located along lake and stream shoreline, in recent clearcut areas near water, in swamp conifer stands, and on snags in marshes and bogs. Occupies a wide variety of habitat types in association with water, primarily near lakes, rivers, and along coastal waters.

Food Habits: Diet consists mainly of fresh fish. Ospreys primarily hover above water, then plunge to catch shallow-water fish. Suckers and panfish are common prey; also consume frogs, ducks, crows, night-herons, and small mammals.

Natural History:

Breeding: Nest at 3 years of age. Clutch size: 2-4 white to pink eggs with brown spots and blotches; laid from April to June. Incubation: 32-33 days. Young fledge in 51-59 days.
Nest: Consist of sticks, bones, driftwood, cornstalks, and other material from marshes; may be 60 feet or more above the ground; can weigh up to half a ton. Nest in loose colonies or singly. Will readily nest in platforms built for them in marshes and flowages, and on the tops of buildings or utility poles. Nest sites may be reoccupied by the same pair year after year.

Management Considerations: The most probable cause of osprey decline is reproductive failure due to environmental pollutants such as DDT. Habitat loss due to lakeshore development and the destruction of nest trees is another limiting factor. The installation of artificial nesting platforms, to replace deteriorating nest sites, has proved to be an effective management tool, especially in recent years on the Petenwell and Castle Rock flowages in central Wisconsin.