In late spring 2012, Harvey Wilson and Tony Brake began a yearlong study of Ospreys in the San Francisco Bay Area. It's where the birds, mostly absent in the 20th century, are beginning a comeback.

Wilson and Brake studied Osprey populations from the South Bay to the tip of San Pablo Bay, but limited their studies to the area west of the Carquinez Strait where the birds have not been seen for many years.

This weekend, to celebrate both the raptor's return and the Wilson-Brake research, the Mare Island Shoreline Heritage Preserve will hold San Francisco Bay Osprey Day.

The two-day event will offer visitors insights into the rare raptors, and organizers say they likely will be easy to spot with the naked eye.

The brown birds have distinct facial markings and white chests, and can dive as deep as six feet into the water for their prey, Wilson said.

Wilson said he and Brake's survey was inspired by the birds' sudden return to the Bay Area and their growing population.Last year, 17 chicks were recorded for the whole San Francisco Bay Area. So far this year, the number has nearly tripled, to 45.

Yet the nest locations may hamper the population growth.

The large birds often choose to nest on tall, man-made objects -- such as cranes and light poles. More often than not, these objects are abandoned and no longer functioning, like a shipbuilding crane where one family nested on Mare Island.

But when those nesting locations are active, it can


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be problematic and dangerous for the birds.

Myrna Hayes, executive director of the Mare Island Shoreline Heritage Preserve, said if the birds nest on active light poles and cranes, the nests may be torn down by building and equipment owners who may not know the birds are protected by the Migratory Bird Treaty Act.

The act prohibits the take down of an active raptors nest in order to protect populations.

"People need to be more informed so they can be proactive about preventing the birds from building a nest," Hayes said.

Allen Fish, director of the Golden Gate Raptor Observatory, said the birds usually settle in man-made locations because they like the area, and there are no tall trees to accommodate them.

Yet the population continues to grow, Wilson said.

Since the birds feed almost exclusively on fish, anything that affects fish population or the birds' ability to catch them will affect the Osprey population.

"I think that the last of the gold rush much has been flushed out, so the water is just clearer," Wilson said. "I don't know if that means there are more fish or not, but the water clarity must affect Osprey fishing."

Wilson said, however, that the reasons for the Osprey comeback are more complex than just water clarity.

"There are three to four factors affecting them," he said. "It's better quality in water, better quality in the Bay Area and maybe a need to move from another location."

Wilsonsaid the Mare Island colony might have branched off from the Marin County colony in Kent Lake.

"That breeding colony started in the 1970s; there's a strong population there," Wilson said. "It's speculation since we have no way to trace them back there, but Tony and I have seen them fly out that way, so it's a good guess."

Wilson also said Bald Eagles nest in Kent Lake, and the birds prey on the Ospreys.

Wilson said he and Brake hope to update the range of the birds and inspire others to study them further -- especially in areas such as nest competition.

"What we noticed while studying the Osprey was that many harass other birds with active nests, we saw that more than we thought we would," Wilson said. "

Raptors usually defend and maintain both their nests and hunting ground, but Ospreys are different because their fish diet precludes hunting grounds, Wilson said.

But Wilson said, they jealously defend their large and complex nests from year to year.

"Success depends on nests. And even if their nests fail one year, they'll stick around to prevent others from using it," Wilson said.

This year more of the overall nests were used and better yet, many new nests were built, Wilson said. Twelve of the 26 nests in the area studied were on Mare Island.

This weekend, Wilson said visitors will get to see both the juveniles and adults.

"They're pretty early this year, so you won't be seeing small chicks in the nest or parents delivering food," Wilson said. "What you can expect to see are a lot of Ospreys flying around the Island, adults and the juveniles they've raised, and a few who are preparing to leave the nest in a couple of weeks."

Contact staff writer Marie F. Estrada at (707) 553-6840 or mestrada@timesheraldonline.com. Follow her on Twitter @MarieVTH.

If you go

Saturday

1. 9:30 a.m. Sierra Club guided hike on Mare Island Preserve

2. Napa-Solano Audobon Society guided tour of nest sites with Robin Leong, 4:40-6:30 p.m. Meet at 3:45 p.m. at G Street and Nimitz Avenue on Mare Island.

Sunday

1. Morning Boat Trip, 8:45 a.m. to 10:45 a.m. Dolphin Charters' River Dolphin, visitors will ride along Mare Island Strait to observe nests, soaring adult and fledgling osprey in the sky.

$20 per person, no children under 8 permitted. Space limited. Pre-registrations must be made by calling 707-249-9633 or emailing myrnahayes@mac.com.

2. Guided outing in Mare Island Shoreline Heritage Preserve

2 p.m. to 4 p.m. and 4:30 p.m. to 6:30 p.m.

Public will see nest sites up close with field guides from the Golden Gate Raptor Observatory, the Audubon Society, the Mare Island Heritage Trust and others.

Leaves from the visitors center, 167 O'Hara Ct. on Mare Island.

For more information, call 707-249-9633

Ospreys at a glance

Scientific Name: Pandion haliaetus

Wingspan: Four to five feet

Special features: Classified as raptors due to their talons, they can move a forward toe forward to aid in the capturing of fish. The also have a modified shoulder joint that allows them to bend their wings back while underwater so they can resurface after a catch.

Diet: 99 percent fish

Habitat: Near lakes, rivers and coastal waterways.

Fun fact: The Osprey can be found on every continent except Antarctica.

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