SILVERTHORNE, Colo. - A sure sign of spring has alighted in Silverthorne, even as snows continue to blanket Summit County. The osprey - a large, white bird of prey - has returned to Colorado's high country from its wintering grounds in South America.
Ospreys can be found all over the world; the birds winter or breed on every continent except Antarctica. Breeding pairs are most common near bodies of water, as their diets consist almost entirely of fish. In Colorado, the birds are especially abundant around Grand Lake, near Rocky Mountain National Park.
Patient bird-watchers in Summit County can observe ospreys in a large nest atop a platform next to Highway 9 near Silverthorne Elementary School. About 15 years ago, when utility lines were rerouted, the pole was left up for the birds, and the Colorado Division of Wildlife constructed the platform on top. DOW officials erected another platform nearby in 2003 to provide a home for fledglings from the original nest. A pair of ospreys took up residence in the new nest about three years ago.
The breeding pair in the first nest has already produced a clutch of eggs, which should hatch sometime next month.
"They do reproduce in Summit County, and you should be able to see some little heads poking out pretty soon," said Sean Shepherd, DOW district wildlife manager. "In that nest, we typically see two [nestlings], but they can have up to four."
Locally, ospreys also make their summer homes on islands in Dillon Reservoir. Since human activity can disturb nesting, boaters are not permitted to disembark on the islands. Ospreys in Summit County usually begin their southward migration in September or October.
The DOW monitors ospreys each summer for nesting success. Populations worldwide were in significant decline in recent decades because of widespread use of the pesticide DDT. In 1983, only about 8,000 breeding pairs spent their summers in the U.S. But a ban on DDT has resulted in a rebound. By 2001, the U.S. population was estimated at 16,000 to 19,000 breeding pairs, according to researchers.
Today, any concern in nesting regions is limited to the potential loss of nest sites to human development. But artificial nesting platforms, like the one in Silverthorne, can mitigate the problem, researchers say.
A new threat has emerged in ospreys' wintering habitats, however. The emergence of fish farms in many South American and Caribbean nations has given rise to conflict between the birds and humans. Osprey are drawn to the thousands of fish farms, frequently swooping in to grab a meal. According to a study by the U.S. Fish and Wildlife Service, about 14,000 ospreys are killed every year by fish farmers in seven Latin American countries surveyed. The agency estimates that total mortality is much higher, given that 21 Latin American and Caribbean nations have fish farms along the birds' migratory routes.