Urban Nature

Ospreys taking a liking to San Francisco Bay

September 11, 2013

 Although ospreys can be found worldwide, until recently there had been no record of their using the San Francisco Bay for anything other than wintering grounds. This nesting season, however, dozens of young ospreys took their first dives through the San Francisco Bay air and caught their first fish in its waters.

Ospreys go south for the winter and spend the breeding season in higher latitudes. In the past, they would pass up the San Francisco Bay to nest farther north.  “If you look at the range route for ospreys now it looks like ospreys come down the Pacific coast and the nesting and breeding range stops somewhere just north of San Francisco,” said Allen Fish of the Golden Gate Raptor Observatory (GGRO).

 But the osprey’s range is clearly changing. Ospreys are unique raptors in that they eat almost exclusively fish and so build their nests right along the waterfront. This makes their nests relatively easy to spot, and in recent years GGRO volunteers like Tony Brake have begun searching for and keeping a record of osprey nests around the Bay.

This osprey nest is atop a power line pole at Point Molate. Photo: Greenbelt Alliance.
This osprey nest is atop a power line pole at Point Molate. Photo: Greenbelt Alliance.

“Tony called me up about a year and a half ago and said that they were doing pretty well finding osprey nests around the Bay,” said Fish. “I knew there were a few – maybe a handful – but I didn’t expect the number that Tony threw at me, which was fifteen in 2012.”

“We’ve had this increasing population for several years now,” said Brake, who along with fellow volunteer Harvey Wilson started an osprey census under consultations with the GGRO.

The first known nest along the Bay shoreline appeared on Vallejo’s Mare Island in 1990, and a single nest remained each season until 2003, when the population began to grow.

 A decade later there are 17 successful nests around the Bay, with a total of 44 fledglings. The nests are concentrated at the mouth of the Carquinez Strait in Vallejo, but a few nests in Richmond were successful this year, as was one in San Francisco.

Osprey arrival

The parents begin arriving and building nests in late February, with incubation starting in late March. The first young hatch at the beginning of May, and in late July they begin fledging, or flying for the first time. The fledglings are currently honing their flying and fishing skills prior to their late-summer takeoff for their wintering grounds, likely in Central and South America.

 The ospreys are usually not found on live trees as one might expect, but rather on top of dead, open-topped trees or on manmade structures like cranes and lampposts, which they find convenient platforms for their extensive nests. That makes this species one of the few that can adapt well to densely urbanized habitats. But trouble arrives when their real estate choices do not match the needs of humans. For example, one of the floating cranes parked at the closed Mare Island Naval Shipyard was removed early in the season, forcing the parents to abandon their nest. And earlier this season, the Port of San Francisco wrecked a nest that was perched on a crane that was needed to bring America’s Cup boats into the Bay.

Ospreys like nesting on top of manmade structures, like this ferry terminal in British Columbia. Photo: Keith Ewing.
Ospreys like nesting on top of manmade structures, like this ferry terminal in British Columbia. Photo: Keith Ewing.

Other feathery couples have been more fortunate. An osprey nest on a lamppost at the California Maritime Academy in Vallejo has been left alone. And Chevron went so far as to build a new nesting platform at the Richmond refinery when the old one became an obstacle to operations.

“It’s encouraging that [Chevron] put a platform up there to deal with it,” said Brake. “Hopefully, that will be an example for other organizations around the area.”

A CSI story

 Experts are unsure why ospreys are suddenly taking to the Bay’s shores. “There are a lot of aspects to this,” said Fish. “It’s kind of an interesting CSI story.”

 While all theories are currently highly speculative, there are quite a few possibilities. The Bay has grown clearer in recent years, the turbidity from the upriver placer mining of more than a century ago having finally settled, leaving the waters clearer and – likely – easier for ospreys to fish in.

Another thought is that the ospreys are spreading down from the Kent Lake colony in Marin, which was first noted in the 1960s and has since grown substantially. There are two parts to that particular theory; it may be that the ospreys are simply moving due to space constraints, but it is also possible that the recent appearance of nesting bald eagles at the lake may have upset the osprey population, inspiring its bay-ward move.

The theories go on. The recent ecological restoration of the Napa River may be playing a role. It is also possible that osprey populations from farther north have grown to such an extent following the ban of DDT that as they migrate over the San Francisco Bay they are choosing to stay.

Historical nesting grounds are in places like Lake Hennessey in Napa. Photo: John H. Wright.
Historical nesting grounds are in places like Lake Hennessey in Napa. Photo: John H. Wright.

 Brake, Wilson, and the GGRO are currently working on a scientific paper to release the news of the Bay’s ospreys to the birding community; in return, they are looking forward to an influx of expert opinions.

“This is all pretty much speculation, so that’s a question that we’re hoping to get when we get the paper out there,” said Brake. “Some people that are fish experts and so forth might have some good ideas on that.”

 Experts are hoping that once the paper is released, the accepted distribution of nesting ospreys will be extended to encompass the Bay.

Can ospreys coexist with humans?

The census will continue next year, but there are many unexplored scenarios as to how the human relationship with the ospreys might progress. Wildlife groups and companies like PG&E that deal regularly with ospreys will gather this month to discuss various interests, particularly how to protect the new population. As the ospreys return in coming years, experts hope to begin tracking some of them to determine where they go and if they return to the same nests. Fish also intends to promote raptor studies in the Carquinez Strait and to get more volunteers involved in order to identify and track osprey nests in the area.

In coming years, perhaps, the ospreys will take more strongly to other parts of the Bay, as the pair in San Francisco has.

“It’s exciting to think it’s not all about the Carquinez Strait,” said Fish. “Ospreys can handle San Francisco, they can handle the weird South of Market industrial areas down at Hunter’s Point that a lot of people have given up for industrial polluted areas and Superfund sites. To have an osprey there is an exciting symbol of ecological wellness.”

Check out this photo slideshow of a family of nesting ospreys on a Whirley crane at Point Richmond.

[slideshow exclude=88361,88364,88365]

 Claire Mathieson is a Bay Nature editorial intern.

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