A Chain Reaction Brought Gulls To The Bay... Now, They Are Eating Other Birds
This story most recently aired in the 1/12/23 episode of Crosscurrents.
A statewide story about L.A.’s water use, Bay Area’s trash, and a whole lot of gulls.
Mark Bowers does not claim to be a gull expert, but he does know that you’re not supposed to call them ‘seagulls.’
“My mom was a birder and she said, ‘Mark, there is no such thing as a ‘seagull,’’ Bowers says. She took him to Half Moon Bay and pointed out to the different species of gulls. “I didn't retain any of it except for the fact that they're different shapes and sizes and colors.”
Bowers manages the solid waste program for the City of Sunnyvale. He’s a soft-spoken man only a month away from retiring after decades in this business. He’s taking me on a tour of the transfer station… and its gulls. A few dozen sit on the roof edge just outside the building, where trash and recycling are sorted and loaded into trucks.
Bowers (and his mom) are right. There are lots of different kinds of gulls. Western Gulls are the ones famous for showing up at Oracle Park. At Bowers’ transfer station, we’re looking at California Gulls. You can tell them apart from the Western Gulls by their yellow legs. And they’re definitely not ‘seagulls because’ – at least historically – they don’t spend their lives at sea. Every year, they migrate from the coast to inland lakes to breed and nest.
But something happened to disrupt that pattern. A few decades ago, humans made changes to one of those island lakes that might have set off a chain reaction causing gulls to nest in the Bay Area. They then end up eating trash at places like Bowers’ transfer station and eating shorebirds, some of which are already on the state’s list of species of special concern.
The first step in the chain reaction happened at Mono Lake, a giant salty lake in the Eastern Sierra, just east of Yosemite, like California’s island ocean. It’s about a hundred and fifty miles from the Bay Area, as the gull flies. Twenty years ago, almost all of the state’s California Gulls species nested here and would migrate out to the California coast in the winter to feed.
Kristie Nelson, a biologist with the science organization Point Blue, surveys the gulls here every year. I go out with her on one of her surveys. We boat out past two large islands, to some smaller rocky islands in the middle of the lake. It’s a beautiful day and the Sierra mountains are our backdrop. But the scene is a little bit more like Hitchcock’s “The Birds.” There are gulls everywhere: standing the islands, hovering directly above us, soaring overhead.
We climb out of the boat and carefully walk uphill to Nelson’s sample plot. Nelson and her field assistant use thumb clickers to quickly count the number of young birds they see within a fenced-in area. And then quickly get back into the boat and motor over to the next island. Gulls fly above us in every direction.
Despite all the gulls everywhere, Nelson tells me that this year’s population could be an all-time low. “It seems really noisy but it’s really a fraction of normal — or at least what used to be normal.
Even if you’ve never been to the Eastern, you may have heard of Mono Lake because it’s one of the sources of Los Angeles’s water. By the 60s, so much water had been diverted south, the lake was starting to dry up. Environmentalists organized a campaign. You might remember seeing those “Save Mono Lake” bumper stickers. The lake level got so low in the 70s that the big island where the gulls used to nest was no longer an island. Coyotes were able to essentially walk out there at the lake’s low points and eat all of the gull chicks. The next year, the gulls picked up and moved to these smaller islands we’re visiting today, which is incredible because these birds usually nest in the exact same spot every year.
“And it was around that same time that a handful of gulls also showed up in a new place,” Nelsons says. A really new place: the San Francisco Bay.
“Do you see how thick that carpet of white is? You can see already just how many gulls are there.” Max Tarjan, a biologist for the San Francisco Bay Bird Observatory, is pointing to an island covered with birds. We’re at Byxbee Park on the Palo Alto shoreline. On one side of us is the Bay and on the other side of us is a marshy area with an island in the middle. On the island is the largest breeding colony of California gulls in the Bay Area. The white we see are the adults, more than eighteen thousand of them, and then in between, blending in, are the grey young chicks.
In 1980, a year after the great gull massacre at Mono Lake, twenty-four California Gulls started nesting in the South Bay. Before then, the species had never nested on the California coast. Scientists don’t know for sure that the birds were from Mono Lake but the timing and circumstance suggest that they were. Since then, the breeding population grew, hitting a peak in 2013 with over fifty thousand gulls. The gulls seem to like the South Bay’s former salt ponds in particular. It’s a similar habitat to Mono Lake. They both have salty water. They have both islands – and in the case of South Bay, levees – for them to nest on. They both have brine flies and shrimp for them to eat.
There’s one thing you probably already know about gulls, though. “They’ll eat really anything they can swallow,” Tarjan says. And there’s one thing the Bay Area has a lot of that Mono Lake doesn’t, and that’s garbage.
This is where Mark Bowers and his transfer station come in. It’s just a few miles as the gull flies from the South Bay’s largest breeding colony. “They find it like a smorgasbord,” Bower says.
When Bowers first started working at this site, it was an active landfill. “There were 1,200 gulls flying around, pooping on everything,” Bowers says. “We were finding chicken bones up on top of the lamppost covers. They’d get the garbage and spread it around and spread disease and generally make a nuisance of themselves.”
When the landfill filled up, the city closed it and opened the transfer station next door. And the gulls moved right over.
Scientists put radio collars on California Gulls to track their movements and discovered that almost all the gulls were visiting waste sites to feed. Waste managers like Bowers have to use all sorts of tricks to try and scare the gulls away. Some bring in falcons; at one point Bowers used fireworks.
“It’s effective but constant,” Bowers says. “You have to constantly do it. I paid a guy full time to drive around all day and just harass gulls.”
His boss heard a rumor that lion urine keeps gulls away and sent Bowers a note telling him to look into it. (Bowers says he still has the note.) So he called the San Francisco Zoo and they just laughed at him. They told him if that was true, then why were there so many gulls in the lion enclosure? “I was breathing a sigh of relief, he says. He thought, “How am I going to put a requisition through purchasing like, ‘One 55 gallon drum of lion urine, please.’”
Bowers borrowed a simple but effective method he learned from a landfill down in Southern California. He installed poles around the edge of the transfer station and strings wires between the poles and the roof building.
The gulls don’t like having to fly around the wires to land and it keeps them away. Until recently, Bowers didn’t have any gulls at his site. But then the transfer station started separating out food scraps from the trash to send to a compost facility and a few gulls made a come-back.
“Throwing your food scraps in with your other garbage is what causes this problem,” Bowers says. He urges people to dispose of food in a green bin or compost if you have access to one.
Scientists found that for some of these gulls, up to eighty percent of their diet comes from landfills. The scientists also discovered that almost all California Gulls in the Bay Area feed at a landfill at some point.
“They like it here,” Bowers says. “Better than Mono Lake.” But not him. “I like it much better in Mono Lake than I do the Bay Area.”
To each his own. The gulls came to the Bay Area to find a safe place to nest and found a great source of food in our trash. When they’re not eating our trash, they turn to something else.
For the next step in the chain reaction, I take a ride with biologist Ben Pearl of San Francisco Bay Bird Observatory. It’s early morning and we’re driving on a levee between two former salt ponds at the Eden Landing Ecological Reserve. California Department of Fish and Wildlife manages the ponds, draining them to different water levels to create different types of habitat for birds. There are ponds still full of water; others are mostly dry dirt fields.
In the distance, we can just hear the river of traffic driving over the Hayward-San Mateo bridge, but it’s mostly the sounds of the marsh here. Shorebirds like American Avocets call incessantly to us. Forster’s terns, a bird that’s like a fighter jet version of a gull, flies overhead.
Pearl surveys for snowy plovers, a grey and white shorebird the size of a soda can. It’s a federally threatened bird. “I think people in a lot of parts of coastal California will know snowy plovers as the bird that shuts down the beaches,” he says.
The bird likes to nest on open flat areas, but as humans took over the beaches, some plovers moved to these former salt ponds. They nest right out in the open, relying on their small size and camouflage to stay hidden.
Pearl and I drive slowly along the levee, stopping every so often for him to scan the field with his binoculars. At one stop, we see a large group of plovers foraging together. At another stop, Pearl points out to me a fluffy black and white plover chick hiding under its parent’s legs.
Right on cue, Pearl sees some California Gulls fly by. He grabs his clipboard to note the gulls on his datasheet as predators. The problem is that California Gulls like this habitat, too and they will absolutely eat a snowy plover given the chance.
The same scientists that tracked the gulls at landfills found that California Gulls are also eating the eggs and chicks of many of the Bay Area’s nesting water birds. The gulls are responsible for half of the predation of American Avocet and Forster’s Tern chicks. Scientists figured this out by putting little metal bands and radio transmitters on the chicks. Then if the chicks got eaten, the scientists could track the radio transmitter – that was either in the beak or even in the stomach of a gull – back to its colony. The researchers found one particular gull that had fourteen Forster’s Terns’ bands in its nest.
Remember, California Gulls haven’t nested in the Bay Area that long. They’re basically the new predator in town. These other birds populations didn’t evolve with them and learn survival strategies.
Complicating the matter, a lot of the old salt ponds are being restored to tidal marsh as part of the South Bay Salt Pond Restoration Project. The levees are being breached and the flat dry areas are being flooded. The project is great for restoring wetland habitat and creating shoreline for public use and flood protection. But it also means California Gulls and birds like plovers, terns, and avocets could end up nesting uncomfortably close together. There’s only so much habitat to go around. “We’ll see large flocks of gulls sometimes foraging right now where plovers are nesting,” Pearl says.
There’s been this interesting cascade of events: Los Angeles diverted water from Mono Lake, which lowered the lake level, which allowed coyotes to walk out and eat the gulls, which might have made the gulls start nesting in the Bay, and then they stayed here because of our food trash, and now they’re feeding on threatened shorebirds! What to make of this?
“Blame it on L.A.!” Pearl jokes. “It’s all their fault!
Maybe we don’t need to assign blame, but it’s certainly an interesting sequence of events to learn from.
Back at Mono Lake, biologist Kristie Nelson tells me that there could be a number of factors contributing to the decline in the nesting gulls at Mono Lake. An invasive weed is taking over the islands, choking out precious nesting space.
“But in San Francisco Bay, there’s dumps and there’s the nests of bay-land nesting shorebirds and there’s probably a lot of food resources for them, so they’ve taken advantage of a situation that was presented to them.” She thinks it’s amazing how well the gulls have adapted.
The gulls have gone from being a species that never nested on the coast to one that’s thriving in the Bay Area. The problem is: they’re sharing space with birds that aren’t as adaptable, like the snowy plover. Maybe the lesson to learn is when we humans change the environment, it almost always sets off a chain reaction that’s going to have implications across species and hundreds of miles.