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An endangered bird’s population is rebounding, but fragile, in Alameda County

on June 3, 2010

Hovering three to four feet in the air above the ocean, a California Least Tern eyeballs an anchovy and swiftly plunges in head-first, spearing the silvery little fish with its long pointy bill. This fish hunter is white and light grey bird with a distinctive black cap, elegant tapered wings and a broad forked tail. While these birds have long been endangered, their population is beginning to bounce back and is finding a home in Alameda County, including the Oakland Estuary and Alameda Air Station—but they remain vulnerable to environmental threats.

The California Least Tern started disappearing in the 19th and early 20th centuries, when the birds were hunted for their feathers, which were used for hats. Before then, “there were hundreds of thousands of Least Terns,” said wildlife ecologist Rob Burton, who works for the Bay Area firm H.T. Harvey and Associates Ecological Consultants. During the last century, the birds’ population shrank even further as waterfront development ate into their nesting territory and brought the birds too close to human populations. By the 1940’s, the terns were either gone or very rare.

When census counts on the California Least Tern—the only subspecies of Least Tern in California—began in 1970, scientists went out to San Diego, Los Angeles and Alameda counties to tally up how many pairs of birds they could find. The total came to just 225 pairs in all of California. That same year the federal government placed the tern on its list of endangered species. “For about 17 years [the terns] just struggled along,” said Burton, before their numbers began to creep back upwards in the late 1908s.

A just-hatched California Least Tern. Photo by gdiproductions via Flickr Creative Commons.

The California Least Terns are sensitive nesters and breeders. They like to mate and nest on abandoned beaches or barren estuary shores near the water during this time of year—in May and June. They don’t actually build nests, but instead find shallow depressions in the ground where they can camouflage their chicks and be protected from sand and wind. In the Bay Area, many nests are found on deserted salt ponds.

Living too close to humans, their pets and their pollutants is a problem for the terns. According to the California Department of Fish and Game, if the terns’ nesting sites are disturbed or become crowded, they will abandon the land. They also are vulnerable to becoming prey for owls, feral cats, crows, raccoons and other predators. In some areas, terns have problems finding enough fish to eat or eating fish that contain toxins like DDT, mercury and selenium.

But after the birds were placed on the federal endangered species list, the population numbers for California Least Terns have rebounded. The California Department of Fish and Game, U.S. Fish and Wildlife Service and several non-profit conservation organizations have been working to protect nest beaches from human development and natural predators. Now, scientists have counted up to 7,000 pairs of Least Terns statewide—and one of the places where these birds are nesting is in Alameda County. “Most activity is down at Ballena Bay Island and Bay Farm Island,” said Burton. “And, for 10 years they’ve been breeding at the Alameda Air Station.” (At one point, the terns even nested between the two airplane runways at the air station.)

“The Navy personnel have done their best to avoid them,” said Burton. “The Alameda colony has been important for Bay population; and Alameda gives 15 percent of [the California Least Tern] population to the total state population.” These terns travel throughout the East Bay in search of food for their young and can be seen around the Emeryville and Oakland mudflats and the Oakland Estuary.

Terns typically lay three eggs and incubate them for 21 days. Both parents take turns caring for the eggs and also feeding the young once they’ve hatched. “These guys are semi-precocious,” said Burton about the hatchlings. “These tiny little guys will be up and running around, but still are still entirely dependent on parents for food.” After three weeks, usually by the end of June, they fledge the nest. By fall, they are heading south for the winter.

A California Least Tern showing off its catch. Photo by Alan Vernon via Flickr Creative Commons.

Even though their numbers have risen since the 1970’s, the California Least Terns are still on the endangered species list because they aren’t reproducing at the rate conservationists want. Over the past couple of years, Burton explains, “Their population has basically stayed level.”

The goal of the U.S. Fish and Wildlife Service is to get the California Least Tern downgraded from “endangered” to “threatened” status, which would mean an improved population. But in order for that to happen, there needs to be at least 1,200 breeding pairs throughout 20 different zones in California, along with other indicators that show the birds will be protected from both human development and predators.

While counts over the past years have shown that the numbers are high enough to merit adjustment of the terns’ status, the other protection requirements have not yet been met. According to biologists at the Point Reyes Bird Observatory based in Petaluma, who have been monitoring the terns at the Alameda Air Station, the birds’ reproductive success at this site has recently declined, possibly because of an either inadequate or tainted food supply.

The U.S. Fish and Wildlife Service warns that if the terns’ nesting sites throughout California are not thoroughly protected, managed and monitored, habitat loss and predators could likely reverse the recent population recovery. Although they’re no longer in danger of extinction, one of the threats that originally caused the Least Tern’s decline—development on their nesting grounds—still exists. The U.S. Fish and Wildlife Service recommends that in addition to continued monitoring and management of the birds, that also new nest sites be created or existing sites, like the Alameda Air Station, be expanded.

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  1. […] But after the birds were placed on the federal endangered species list the population numbers for California Least Terns have rebounded. The California Department of Fish and Game U.S. Fish and Wildlife Service and several nonprofit … […]

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