RICHMOND -- The osprey, a fierce and powerful fishing bird, used to be just another San Francisco Bay shoreline visitor, flying over and feeding without setting down roots.
These days, however, the acrobatic divers are becoming natives, constructing elaborate stick nests on cranes, poles or other man-made structures.
Ospreys nested in 21 places around the bay shorelines this year and 17 last year, up sharply from a single active nest in the early 1990s. Most of the new nests are in the East and North Bay.
Before that, ospreys steered clear of the bay, preferring to nest in areas far north of the Bay Area, or inland, especially near lakes.
Scientists aren't sure why the raptors have made the move. They speculate it may have something to do with changes in bay water quality, clarity and abundance of fish.
Or it may have something to do with bald eagles -- another species on the upswing -- crowding ospreys out of their prime nest spots near lakes in the North Bay.
"We don't have all the answers, but the nesting territory is a significant change that has occurred in a relatively short period of time," said Tony Brake, a volunteer bird monitor with the Golden Gate Raptor Observatory.
Brake and fellow volunteer Harvey Wilson noted the trend as they studied nest sites over the past three years.
"They are very charismatic birds with tremendous diving skills," Brake said. "And fortunately for us humans, ospreys are much more tolerant than many large raptors of people."
With wingspans of 5 to 6 feet, the slender ospreys soar over water to spot fish, then nose dive from up to 100 feet in the air to pluck up prey as deep as 3 feet below the surface.
Although osprey populations were once threatened because of DDT contamination, the birds now prosper in many areas, living on every continent except Antarctica. They migrate through or winter in the Bay Area, and they have nested in the past around Lexington Reservoir near Los Gatos and Kent Lake in western Marin County.
Lately, they have shown a preference for the East Bay shoreline.
On clear mornings, Brake said, he gets a little mental lift when he peers through a spotting scope at his Point Richmond home and sees two juvenile ospreys in a nest a mile away, built on a historical shipbuilding crane in Richmond.
Earlier this week, he watched up close as a daddy osprey dropped striped bass and other fish to the two young birds making loud cries that reverberated along the shoreline.
"They beg very loudly, and they don't stop calling even when they're eating," he explained. "Ospreys are very persistent."
Brake watches and takes notes on nest building, breeding, feeding and fledging. In early July, though, he helped a San Rafael wildlife center worker remove fishing line cutting into the leg of a young bird. It recovered nicely and has started to fly.
Most of the new nest sites are in or near industrial areas in Richmond or Vallejo -- spots with cranes, poles or platforms that provide substitutes for a tree snag or crown.
The city of Richmond built a platform on a pier not accessible to the public, and ospreys nested there.
Officials at the Chevron oil refinery in Richmond also built a platform to lure ospreys to nest and steer clear of their original destination: a power pole that could have electrocuted them, Brake said.
The growing population of ospreys along the shoreline has spurred new opportunities for cities or industries to build platforms as safe nesting spots, said Cindy Margulis, executive director of the Golden Gate Audubon Society.
"It's a wonderful way for people to connect to wildlife," Margulis said.
Shoreline property owners also need be on the lookout for osprey nests. It is a violation of international migratory bird treaties and federal law to harm chicks in a nest, said Allen Fish, director of the Golden Gate Raptor Observatory.
The Port of San Francisco last year ruffled birders' feathers when it removed an apparent osprey nest from a crane needed for the America's Cup yacht races.
The U.S. Fish and Wildlife Service investigated and found no legal violations by the port, which said the nest was empty, Rebecca Rocca, an agency supervisor, said in an email.
Fish advises property owners who find osprey nesting activity on their property or equipment not to remove nests without first consulting with the state Department of Fish and Wildlife.
"Osprey can put together nests very fast," Fish said. "If you have a place along the shoreline that could attract them, you could end up with an osprey nest in February or March."
Contact Denis Cuff at 925-943-8267. Follow him at Twitter.com/deniscuff
Osprey: A large raptor that is the only North American hawk that feeds almost exclusively on fish.
Wing span: 59 to 70 inches.
Adult weight: 50 to 71 ounces.
Fishing skills: Ospreys catch fish on one of every four dives, making a catch about once every 12 minutes of fishing.
Frequent flier: An osprey may log 160,000 air miles in migrations over its lifetime of 15 to 20 years.
Nests: Contains two to three eggs on average. Stick nests can be 10- to 13-feet deep and three- to six-feet in diameter -- easily big enough for a human to sit in.
Risks: Ospreys build their nests with sticks and other scooped up materials, including fishing line. Unfortunately, the line can wrap around and injure the chicks.
To view a live video feed of an osprey nest in Montana, visit: http://cams.allaboutbirds.org/channel/27/Hellgate_Ospreys/