Photos began cropping up on social media around January 25: seabirds, ensconced in glistening sand, crumpled grotesquely in the golden hour light and strewn up and down the beach in Venice.

“I’ve passed   about  10  of these dead birds in just a  few minutes,” a Twitter user named Laura Lee reported. “What’s going on?”

That has been the question at the heart of a weeks-long investigation that was launched in the hours after reports began to flood in of a potential die-off. 

The birds at the center of the investigation are known as Brandt’s cormorants. They’re large, black marine birds with curved necks and long, hooked beaks. They feed, nest and spend their lives along the Pacific coast between Mexico and Alaska.

“They’re gregarious birds. They breed in a big colony,” International Bird Rescue Director of Research and Veterinary Science Rebecca Duerr said. 

When the bodies of dead cormorants first began turning up, a range of theories emerged as to what caused the birds’ deaths. Suspects included pollution, sea temperature rise, bacteria from heavy rain run-off, starvation, avian influenza (bird flu) and parasites.

“Most of some of the more recent, notable die-off events have been linked to human activity,” Luke Ginger, Heal the Bay water quality specialist, said the first morning the birds were reported. Ginger said at that point they knew no details, but that a 2019 marine mammal die-off was thought to be linked to “changes in the foodweb structure of the ocean, because when the water warms up, that kind of throws off different things like fish migration, that throws off spawning times for fish. And, as a result, the entire food chain is out of synchronization.”

Coliform bacteria washing out of storm drains was another early theory, quickly debunked by LA County Public Health’s chief veterinarian Karen Ehnert. 

“It’s just one species of birds involved; there’s not been any fish die-off or anything,” Ehnert said. “There’s another issue with some of our beaches [that] have been contaminated with coliforms in the past, which can happen after rains, where there’s runoff out into the ocean and it can be contaminated with that. But we’ve not heard of any humans being ill or anything — just this one type of bird.”

Social media users surmised it could be bird flu, which at the time was cropping up all over Europe, particularly in Germany and the United Kingdom. Duerr said that, fortunately, no Brandt’s cormorants have been found with the ailment.

Ehnert floated the idea of a parasitic protozoa outbreak, but said in previous instances the protozoa did not kill Brandt’s cormorants; it just made them noticeably ill.

“During an outbreak back in 2019, the birds were actually neurologic and stumbling and everything and were sick,” Ehnert said. “They weren’t just found dead. So, we can’t absolutely say that’s what it’s going to be. That’s why we need these test results, because we can’t make any judgment until we have some sort of cause of death.”

So, teams went out to collect as many specimens as possible and begin the investigation. That team included specialists from the Los Angeles County Department of Public Health, California Department of Fish and Wildlife, Los Angeles Animal Services, Los Angeles County Beaches and Harbors and the nonprofit International Bird Rescue, who have been counting, collecting and analyzing the remains of dozens of dead birds that have washed up on beaches from Marina Del Rey to Malibu since the start of the year. Initially, the remains of about 100 birds were counted, although researchers speculated there could be many more.

Duerr said counts can be tricker along Los Angeles beaches that are “groomed” with machinery and lack volunteer beach monitors, often called Beach Watch, who are more prevalent in other parts of the state. 

“Consequently, that information in much of the state gives us a background level of stranding, like what’s normal mortality,” Duerr said. “It lets us spot when there’s a spike in death of a certain species. That’s very difficult down here, especially on those west-facing beaches in LA, just because they are groomed.” Simply put, cormorant remains could be collected amid the cigarette butts and driftwood and discarded without ever being counted.

But where feasible, LA County employees have stepped in since the die-off was first reported.

“Beaches and Harbors staff are going out and collecting birds to properly dispose of them,” Ehnert said. She added: “At least in the areas where Los Angeles County Beaches and Harbors is in charge, there’s a pretty good count. I can’t say beyond our area, and I don’t know, actually, the whole jurisdiction, because they’re not our department. 

“If they go all the way up into Malibu, there’s a lot fewer staff, and areas that are a little more remote,” Ehnert continued. 

“We’re not specifically looking for these, and I don’t know any groups that are,” California Wildlife Center veterinarian Dr. Guthrum Purdin said of the Malibu beaches, “but because the beach lifeguards are very aware of what’s on the beaches, they may be picking some of them up, and they are certainly picking up some for us — live and not alive.”

LA County Beaches and Harbors collected 35 dead cormorants on Jan. 27 and another 18 the following day, all on Venice Beach. Between Jan. 29 and Feb. 24, another 60 or so bodies were collected.

Cormorants are social. They eat together — with a whole colony often going after schools of fish all at once — and nest together, which means they are also susceptible to the same environmental factors. Unlike seagulls and sandpipers, they spend most of their time out on the water or on offshore islands, so they aren’t the type of bird you’d see eyeing your beach picnic. They also eat fish exclusively.

“These are really interesting birds, because they’re diving birds, but their wings are really not waterproof. So, one of the things the public will see as they’re out on jetties and on piers is they’re spread out to dry,” Russ Curtis, a spokesperson for the International Bird Rescue, described. “[Their wings] get waterlogged if they don’t dry them out.”

Spend enough time on the Southern California coast in the winter and you’ll probably see a Brandt’s cormorant at the end of a pier. The population grows in size during winter months when northern waters are less hospitable.

“My understanding is their population is fairly stable,” Krysta Rogers, senior environmental scientist for the California Department of Fish and Wildlife, said. “They’re not classified as threatened or endangered. So, it appears that their population overall is pretty healthy, and they’re relatively abundant along the coast.” 

Rogers said Brandt’s cormorants are known to breed on the Channel Islands. Neither Rogers, nor Duerr, nor the other specialists quoted in this story were able to provide an approximation for the total population number in our area, but Rogers estimated a die-off of this size would be “locally important … significant, but probably not overly significant.”

Still, when specimens began to trickle into the International Bird Rescue center in San Pedro, veterinary scientists were paying attention. 

“What happens when there is a big die-off is that whatever local wildlife center ends up taking in the animals is like, ‘Oh my God, we’ve got something going on here,’” Duerr said. “So, the animals get transferred to us and all of a sudden we get a big influx of something.”

But whereas many times die-offs involve a mix of live and dead specimens, the January event was exclusively dead birds — dozens of them, in varying states of decay.

“Having a live animal that has a problem is much more informative than a whole bunch of dead animals that are decaying on the beach,” Duerr said. “It gives us a much better idea of what was going on with all those other ones.”

Unfortunately, the specimens collected in January were already rapidly deteriorating.

“Generally, these were dead birds found on the beach and they had been in the water, so their condition is not all that great,” Ehnert said. 

The results of the first wave of necropsies appeared to bear that out. 

Roughly five weeks after the first samples were sent out for necropsy, results were in — kind of. 

The first birds sent into UC Davis’ California Animal Health Food Safety Lab in San Bernardino were necropsied but no obvious cause of death was found.

Then, in February and early March, more Brandt’s cormorants were found washed up on local beaches. But these birds were alive, if very ill.

“Initially, we only were hearing about dead birds being found, but there were no reports of ill birds. That has changed recently and there have been some cormorants with neurologic symptoms,” an emailed statement from the Department of Public Health described on March 3. It later added: “The investigation is ongoing, but it is appearing that early on, the deaths may have been related to starvation in juveniles, and now we are seeing birds becoming ill with a protozoal infection.”

What are the odds of back-to-back unrelated die-offs of the same species in the same location? Not totally out of the question, it turns out. 

“Pretty much with any wild animal, if they have a good breeding year, the following winter is often hard on the young animals of the year,” Duerr said. Less life experience and more competition can result in starvation-related die-offs.

“Adults are generally more competent at being alive,” Duerr said. “They have extra tricks up their sleeve when things get tough.”

But the degraded specimens make it difficult to perform thorough necropsies on cormorants’ brains — the most direct way to detect many potential causes of death, including the protozoic parasite sarcocystis calchasi, which Duerr, Ehnert and Rogers all quickly zeroed in on as the most likely culprit. 

“Brains turn into goo pretty quickly when they’re degrading,” Duerr said.

So, when seriously ill birds arrived at the International Bird Rescue — birds that refused to eat food or medicine, in serious neurological distress — virtually all died or were euthanized, and Duerr began preparing them for dissection and necropsy. Based on symptoms, the team believed it was likely the birds were infected with sarcocystis calchasi, a type of microscopic parasite known to exist locally in two types of birds, known as “intermediate” and “definitive” hosts. The “intermediate” hosts are rock pigeons, which are prevalent virtually everywhere in the United States. The “definitive” hosts are two types of local raptors: red tailed hawks and Cooper’s hawks. Generally speaking, the parasites live in cysts that grow in the muscles and brains of rock pigeons, which are preyed upon by hawks. When hawks eat infected pigeons, the parasites enter their digestive system and eggs are released as excrement, which find their way back into the pigeons (not known to be picky eaters).

In that way, the parasite’s life cycle continues. Sarcocystis calchasi, like all parasites, does not want to kill its host. Its goal, if a protozoa can have a goal, is to stay in its cycle.

So the idea that the protozoa has found its way out of that cycle and into what Rogers called a “dead-end” host, Brandt’s cormorants, is still a bit of a puzzle for researchers. 

However, results proved the hypothesis true — most (though not all) of the later February and March specimens were found infected with the pathogen.

Duerr posited that the cycle could be: eggs in raptor waste flushing into the ocean, being eaten by filter-feeders (like mussels), those being eaten by fish, which are then eaten by Brandt’s cormorants. At that point, the cycle ends and the parasites eventually die.

Sarcocystis calchasi was first found in Brandt’s cormorants in 2019, and it has reappeared in the species every winter since, making 2022 the fourth consecutive year.

“Why Brandt’s cormorants are more susceptible to it than another species of cormorant or another species of seabird, we don’t know,” Rogers said. “It could just be that we haven’t detected it yet. Or it could be that they’re just not susceptible to it.”

Duerr said pathogens traveling through the food chain often take “micro pathways” to infect a particular species, since some fish, for instance, are only preyed upon by certain birds. That could be the case for this outbreak.

How big of a factor is rainfall in the passage of the parasite to local cormorant populations? In the first winter sarcocystis calchasi was found in local Brandt’s cormorants, monthly rainfall went from 90 millimeters in December 2018 up to 217 mm in January 2019, 392 mm in February, 128 mm in March and 3 mm in April.

“These patterns of rain that we’ve been getting where it’s just a real burst over a relatively short period of time, rather [than] drawn out over months, could very well influence the severity of these outbreaks, because the severity is dependent upon how many parasites a particular individual ingests,” Rogers said. “So, if an animal, theoretically, just ingests one to two of these parasitic eggs, it may not cause any problems, whereas if they get a really large dose, all at one time, then that’s more likely going to develop into clinical disease. And so it could be that these very significant rain events that we’ve been having these last few years do play a role in the amount of water that is getting flushed out from the city.” Rogers said that theory was still speculative.

Regardless, the possibility of sarcocystis calchasi infection in humans or pets who frequent local beaches is incredibly minute, if not totally out of the question. 

Rogers said the parasite is known only to circulate in avian species; parasites adapt to live in certain environments, and are very unlikely to jump from birds to, say, mammals. Still, the existence of these pathogens is a good reminder to stay out of the ocean following heavy rainfall.

As for Brandt’s cormorants, they’re out of luck: “As long as we have rock pigeons, it’s probably something we’re going to see at least periodically in these cormorants,” Rogers said.