It’s osprey watching time in Sonoma County

A sharp call whistles down the river, then another is echoed by the same species of black and white raptor.

Flying in loops 100 feet up while peering through the ripples on the Russian River’s surface, the two osprey do their astral dance, soaring, circling, turning on a dime and swooping.

In flight, the osprey has arched wings and “drooping” hands, piercing the river valley with a series of clarion whistles.

One pauses, kiting, then folds his wings against his body and drops feet first with a satisfying splash beneath the surface of the water. If he calculated correctly, he will come up with a fish gripped torpedo-like in his talons and bring it home to mama and the chicks.

The way of ospreys (and all birds) is for males to court and compete for females. After studying the field, it’s up to her to make a choice.

The season for that mating ritual is set to begin, with birds that have flown as far south as Chile returning from their migration around President’s Day to stake claims to their arboreal turf, For the most part, they prefer to move into last year’s condo.

Ospreys favor rivers, lakes, wetlands and coastal marshes, with adult males arriving in their nesting territories a few days before the females. Their nests are in the general area of the nests from which they fledged and were home the year before.

As long as they’re near water, osprey will build nests on dead trees, rock outcrops, buoys, electric towers, pier pilings and man-made platforms.

Their nests are a bulky cauldron of branches, grasses, vines, cones and bones often five feet in diameter and two- to seven-feet thick. The adult pair regularly do nest maintenance, replacing what has been blown away or fallen and patching the lining.

Watching them accomplish this can be exciting, as they may be seen breaking limbs off neighboring trees by flying at branches with enough force to cause them to snap off upon being struck by the four-pound bird’s talons.

The osprey is diurnal and usually about 24 inches in length with a 71-inch wing span. It’s brown on the upper parts and predominantly grayish on the head and underparts with a black eye patch and wings. Males on the hunt for a long-term mate need to be more attractive, while females’ plumage may be a bit less showy. But when it comes to size, as it is with all raptors, the females are designed to be a bit bigger in order to accommodate their eggs.

The male is committed to keeping the family fed throughout the egg laying process and the three to six week incubation period.

His catches weigh from ?4 to 68 ounces, and the nonskid padding on osprey foot bottoms keep the slippery fish from wriggling free. Also, securing the fish is made easier by osprey talons that are rounded (coming together in a circle) instead of grooved, yet another sign of adaptive engineering.

Osprey eggs hatch at irregular intervals and, considering their chances of survival, the first chick to hatch has an obvious advantage. Should they all survive, a brood of three reportedly requires six pounds of fish a day.

On the Russian River, tops of snags are plentiful, generally have good views of the River and are thus favored locations.

Many of the nests are easy to spot. At one location about a mile west of Duncans Mills, as many as 15 nests may be seen on the south bank of the river. The overlook at the river’s mouth also is an excellent observation point for osprey as well as pelicans, gulls, terns and the seals that ply the waters of the estuary.

One of the most visible nests is in a big redwood snag on the river, opposite Duncan Road on the south side of Highway 116. Parking spaces are easy to find; you may walk up Duncan Road until you’re looking down at the nest.

A couple has been raising broods there for several years, and a few early sightings have been reported.

Come November, these fish hawks will again be yearning for Argentina and Chile, but a few birds will remain to work the waters of the Russian River and down the Pacific coast to Bodega Bay.

The uninitiated often mistake the osprey for a bald eagle, but eagles are considerably bigger, have massive beaks and hold their 70-inch wingspan parallel to the ground when flying. Eagles are opportunists, and whenever they can, they’ll rob an osprey of its catch.

Now is a good time to keep a pair of binoculars handy and grab a close-up view of one of nature’s true wonders in action in your own backyard.

Contact River Towns Correspondent Stephen D. Gross at sdgross@sonic.net.