ON A PACIFIC NORTHWEST summer day, when the sky was blue for 16 hours, I got a call from a local kayak guide who offers nature tours. “I found a dead tern,” he said. There were nearly two thousand Caspian terns nesting on a small sandbar across the bay from my town, Port Townsend, Washington. Their raucous calls, bright red bills, and spectacular dives for fish thrilled everyone from local photographers to tourists on the ferry boats.

“Probably not a big deal,” I told him, “unless you start seeing more.”

Within two hours, he found a dozen more. He called the Washington Department of Fish and Wildlife. Some department officials, who happened to be nearby in a boat, came over to check it out. They collected 35 fresh tern carcasses and right away suspected that highly pathogenic avian influenza (HPAI), caused by the H5N1 virus, was the culprit. Within days, results from the lab confirmed it.

This was only the second large avian flu outbreak documented in Washington in wild birds, and the first-ever to happen during the breeding season.

AVIAN FLU VIRUSES have their origins in wild birds, where they have usually been low pathogenic, showing little or no clinical signs of illness. The only known exception was an outbreak that killed over a thousand common terns in South Africa in 1961. But in domestic poultry flocks, HPAI has been wreaking havoc for almost a century. In fact, “highly pathogenic” is defined by having a mortality rate of over 90 percent in domestic poultry. What happened in 1961, and again in 1996 in China, is that new variants evolved in domestic birds and went back into wild birds with deadly consequences.

In 2005, the deaths of 1,500 bar-headed geese at Lake Qinghai, China, marked the first mass mortality event from avian flu in wild birds. In the ensuing decade-and-a-half, the virus spun off more mutations and continued to be detected at low levels in wild birds from China to Africa, the Middle East, and Europe.

In the summer of 2021, we entered a new era — a highly pathogenic variant struck nesting seabirds, beginning with a great skua colony in Scotland’s Shetland Islands. Ninety percent of the adults died. This was followed by mass mortality of barnacle geese, gannets, gulls, and terns elsewhere in Europe. By winter 2021, the virus had spread to Asia, Africa, and the Americas. An estimated 8,000 common cranes died in Israel. Significant mortality was also documented among gulls and eiders in New England, murres and gannets in Atlantic Canada, great white pelicans and royal and Caspian terns in western Africa, African penguins and cape cormorants in southern Africa, demoiselle cranes in India, and hooded cranes in Japan. By the next winter, in November 2022, it reached Peruvian pelicans in South America. The exact route — and species — of dispersal remain speculative.

Outbreaks have now been documented in wild birds on every continent except Australia and Antarctica. A recent report by the United Nations Food and Agriculture Organization (FAO) described it as “causing unparalleled mortality of wild birds and mammals worldwide.”

While species that breed in dense colonies, or that migrate and winter in large flocks, are most vulnerable, birds and mammals that scavenge dead and dying birds have also been impacted. Avian flu has been detected in eagles, hawks, seals and sea-lions, dolphins, bears, felines, canines, skunks, raccoons, and bears. Of greatest concern were the deaths of 21 endangered California condors, out of a total population of 561, in May this year. The US Fish and Wildlife Service is currently experimenting with a vaccine to protect them.

The FAO report noted that “important breeding colonies on oceanic islands are at risk.” By that, they mean seabirds. For many seabird species, a large percentage of their entire population nest at only a few sites, or even at a single colony. On the Pacific Coast, for example, over 95 percent of the world’s Heermann’s gulls come from tiny Isla Rasa in the Gulf of California. The same can be said for many species of albatrosses, shearwaters, petrels, and penguins.

These seabirds, as the FAO report says, are “already under threat due to the consequences of climate change and reduced food availability.” Die-offs due to oceanic heat waves and dramatically shifting fish populations have occurred worldwide. This has caused many seabirds to fly farther in search of food. Caspian terns appearing in northern Alaska and Nazca boobies, which breed exclusively on the Galapagos Islands, showing up in the Pacific Northwest, are two examples. As I write, I am hearing of several hundred short-tailed shearwaters in the Northwest Passage, presaging yet another species spreading between the Pacific and the Atlantic through a warming Arctic.

Ominously, three of them were found dead and tested positive for avian flu. Short-tailed shearwaters breed in New Zealand, which has yet to be affected by the virus.

LIKE MANY SEABIRDS facing compound problems, the terns across the bay from my town had already been reduced to this single colony in Puget Sound. Most had been hazed away from islands in the Columbia River as they were preying on salmon, which in turn are locally endangered due to a staircase of dams on the river. The terns moved to the rooftop of a warehouse in Seattle. During the massive Pacific Northwest heat dome of 2021, the roof became unbearably scorching, causing chicks to hurtle themselves into the traffic below. Since then, building owners have hazed the birds away from that location as well.

The terns I was watching were a colony of refugees, continually in search of a new home due to a cascade of assaults. When they first came in large numbers to the island in 2022, they suffered catastrophic breeding failure due to a combination of human disturbance and predation by gulls and coyotes.

This summer, we were ready to protect the colony from wandering campers and dog-walkers with a volunteer docent program. Instead, just as a thousand eggs were hatching, we found ourselves protecting dogs and humans from a virus carried by dying terns. (Yes, HPAI can affect humans, though such cases are extremely rare.)

By the end of the summer, avian flu had taken 80 percent of the adults and over 500 chicks. For long-lived species like seabirds, the loss of adults is especially significant. Most Caspian terns, for example, live 10 to 15 years. This means they only need to reproduce themselves once every 15 years or so to sustain their population. It also means that, if they lose an adult, it can take many years to replace them. Some seabirds, such as albatross, live over 60 years.

Research into the rapidly evolving strains of H5N1 is now on-going across the world. Seabird monitoring has revealed some hope. After the initial outbreak at the skua colony in 2021, the virus spread to other colonies in Scotland the following summer, ultimately reducing the skua population by two-thirds. However, surveys in the summer of 2023 found that the population had remained the same as in 2022, and no further outbreaks were observed. Similarly, a recent survey of heavily-impacted gannet colonies show acquired immunity in the following year. Additionally, mortality was lower in colonies where nests were more spread out. This can help protect the population from large declines in a single year.

On my last visit to the terns across the bay, there were still nearly 350 adults. Listening to the cacophony and looking through my scope, I could see a few dead adults, and some that appeared sick. Higher up on the island, however, terns were flying into the colony carrying fish to begging chicks. We estimated that about 20 percent of the adults survived. That sounds pathetic, but it’s a new beginning. And, despite the carnage, it looks like this colony may produce more young than last year. So that’s another beginning. At low tide, fledges were making their first flights to an adjacent sandbar. They can’t get away fast enough.