A gull species that didn't nest anywhere near San Francisco Bay a few decades ago now numbers in the tens of thousands here year-round — and its explosive rise, fueled by the region's landfills, is squeezing some of the Bay's most endangered birds.

KQED's California Report Magazine this week revived a question Bay Area naturalists have wrestled with for years: are the California gulls swarming the South Bay native or invasive? The honest answer, as KQED put it, is "maybe both." The California gull (Larus californicus) is a genuine Californian — once so scarce, per the compiled species record, that it was listed as a state Species of Special Concern — but its South Bay colony is a modern, human-made phenomenon. That distinction matters, because the same salt ponds and garbage that let the gulls thrive are now a pressure point for threatened shorebirds nesting in the same shrinking habitat.

For all the ice-cream-cone thievery that earns them their bad name, California gulls were, within living memory, a conservation worry rather than a nuisance. Their historic breeding stronghold at Mono Lake in the Eastern Sierra was declining, and the birds carried protected status in the state.

Then they found the Bay. According to the compiled species record for Larus californicus — drawn from USGS and published South Bay colony research — California gulls did not historically breed in San Francisco Bay at all. Beginning in the early 1980s they colonized the South Bay's remote salt-production ponds and levees — habitat left behind by more than a century of industrial salt-making — and the population exploded. That same record puts the South Bay colony at fewer than 1,000 breeding birds in 1982 and more than 33,000 by 2006. What powered the boom, by that account, wasn't wilderness but infrastructure: a reliable, year-round subsidy of trash at landfills serving San Francisco, San Jose and communities up toward Sacramento.

The result is a bird that is native to California yet behaves, locally, like an introduced predator. According to the same compiled account, large resident flocks now prey opportunistically on the eggs and nestlings of other birds sharing the South Bay flats — including the western snowy plover and the California least tern, both protected species clinging to small nesting footholds along the Bay, and less-threatened neighbors such as black-necked stilts, American avocets, and Forster's and Caspian terns. Wildlife managers have responded by trying to shrink gull habitat and disperse the enormous flocks, an awkward campaign against a native species that is simply doing what gulls do.

Here a distinction matters, because the Bay has two year-round resident gulls that are easy to confuse. The one with the truly villainous reputation is not the California gull at all but the western gull — a larger, pink-legged species, a different bird entirely. "Much that is good and all that is evil has gathered itself up into the Western Gull," ornithologist W. L. Dawson wrote of that species in his 1923 classic Birds of California, describing a bird that "asks only two questions of any other living thing: First, 'Am I hungry?' … Second, 'Can I get away with it?'" Biologists still speak of westerns that way. "Western gulls can be very challenging," PRBO Conservation Science biologist Russell Bradley told Bay Nature — again, of the western, not the California gull — recalling fieldwork on the Farallones where researchers wear hard hats against dive-bombing adults.

The California gull is the quieter cousin, and by the numbers the more consequential one. It didn't remake headlines by mugging tourists on the wharf; it remade an ecosystem, reading a landscape of salt ponds and dumps and turning it into one of the West Coast's great urban seabird colonies.

That is the tension KQED's Bay Curious team, reporting from the salt ponds, set out to untangle — why "tens of thousands of birds ditched breeding grounds in the Eastern Sierra and have permanently relocated to the Bay." The ecological bookkeeping is still open. As the salt ponds are restored to tidal marsh under the sprawling South Bay Salt Pond Restoration Project, the flat, predator-friendly islands the gulls favor are changing, and no one is certain whether the colony will hold, shift, or shrink. For now, the gulls remain the Bay's most successful opportunists — a homegrown species whose comeback is a genuine conservation success and a genuine conservation problem at the same time.