Nearly 100,000 birds dead in botulism outbreak linked to climate change, water diversions

By Douglas Main

An ongoing outbreak of botulism, a bacterial illness that causes muscle paralysis, has killed more than 94,000 birds at Tule Lake National Wildlife Refuge in Northern California, the worst such outbreak at the lake ever recorded, according to federal scientists.

Affected birds often cannot control their muscles and often suffocate in the water, said biologist and ornithologist Teresa Wicks, with Bird Alliance of Oregon, who works in the area. “It’s a very traumatic thing to see,” Wicks said.

Though local in scale, the outbreak and catastrophic die-off are tied to global problems including declining wetlands, increasing demand for limited water resources, hydrological diversions, and a warming climate.

These kinds of outbreaks can happen around the world and the phenomenon seems to be on the rise, according to Andrew Farnsworth, a scientist at the Cornell Lab of Ornithology who studies bird migration. 

“Given warming temperatures, droughts, then intense periods of rain followed by drying… the hallmarks of climate change are all over this,” Farnsworth said.

The pestilence is caused by a toxin produced by a specific type of bacteria (Clostridium botulinum) that thrives in the area’s warm, stagnant, low water levels. Botulism can also affect people, though no human cases have been reported in this instance. Other outbreaks have been reported around the world, but generally cause far fewer deaths. A botulism outbreak in 2020 caused by similar conditions killed an estimated 60,000 birds at Tule Lake. 

The Klamath Basin, of which the refuge is a part, has been disrupted by man-made dams and irrigation canals for over a century. The developments and diversions eliminated more than 90% of the area’s wetlands.

Tule Lake is an ancient water body, whose levels swelled and ebbed, but always remained, for hundreds of thousands of years. Historically, the lake and nearby wetlands would fill with water during the winter rains. Now, the water supply comes almost entirely from irrigation canals.

In 2021, the lake dried up entirely for the first time in recorded history. In October 2023, local drainage districts and the US government’s area Bureau of Reclamation released water to the lake. Birds quickly returned.

But this summer, water levels declined again and avian influenza killed hundreds of birds, and then the botulism outbreak began. 

In response to an outcry from advocacy groups, the bureau delivered more than 5,000 acre feet of water to the lake, slowing the outbreak. But no more has been delivered, and by late August, more than 1,000 birds were dying every day.  Similar numbers died throughout September and into October. The death toll is almost certain to surpass 100,000.

Scientists say they are hopeful the outbreak will cease once cold, freezing weather returns, hopefully later in October.

“Although botulism is naturally occurring, the scope of this outbreak is disheartening,” said John Vradenburg, supervisory biologist with the Klamath Basin National Wildlife Refuge Complex. “Wetlands are declining across the western U.S. due to changes in the climate, alteration in ecosystem function and increasing demand on limited water resources.”

Tule Lake and nearby waterbodies provide a vital stopping point on the Pacific flyway, which millions of birds migrate along every year. According to Ducks Unlimited, a conservation and hunting group, the numbers of birds using the flyway in California are down 25% this year compared to 2023, and down 30% from recent historical averages. 

Many types of migratory waterbirds and shorebirds have died, including northern shovelers, pintails, and American widgeons. Mallards, gadwalls, coots, sandpipers, and wading birds including avocets, black-neck stilts, and long-billed dowitchers have also been killed.

Wildlife rehabilitators Marie Travers and January Bill and their organization Bird Ally X have treated about 1,500 affected animals in a field hospital for birds, and successfully released over 900. There is no cure for the botulism toxin, but given supportive care and a low-stress environment, most birds can recover over the course of a week or so, they said.

The group had to quit collecting birds to treat on October 5 with the start of duck-hunting season.

In the long term, a return of wetlands and a more sustainable, natural flow of water will be necessary to prevent this from happening again, Wicks said. Several large dams have now been removed from the Klamath, and discussions are taking

place about improving the system, which mostly means letting it function how it did before human meddling.

The conversations “make it feel more hopeful that we can find a solution to the problem,” Wicks said.

As it is now, however, Tule Lake and surroundings wetlands have the most junior water rights, below agriculture and other human uses. “Such an important refuge for migratory birds has just been allowed to disappear” at times, Bill said.

“There aren’t enough people that can advocate for the birds getting the water they need to stay alive,” Travers said.

(Featured image by Dave Menke/USFWS.)