A crossing guard first noticed the disoriented Canada goose wandering in a Corte Madera intersection, seemingly unable to see. Then a bystander helped corral the bird into a dog crate and brought it to WildCare. It was late September, and the animal hospital was already prepared for an influx of avian flu patients. Dr. Juliana Sorem examined the goose at a new outdoor triage area. She was concerned to see that its eyes were cloudy: a corneal edema is a key symptom of the virus in Canada geese. Erring on the side of caution, she euthanized it and sent it to the California Department of Fish and Wildlife for testing. A month later, WildCare got the word: Marin had its first confirmed case of highly pathogenic avian influenza, the bird flu strain that is killing fowl by the millions nationwide and is likely widespread among wild birds in the county. The virus is more potent and fast-spreading than ever before in the United States and has afflicted a wider variety of species. “In the past it mostly affected domestic poultry and a few wild birds,” Dr. Sorem said. “This time around, it is decimating wild birds, especially swans, geese, pelicans and raptors.” The virus has been detected in wild birds across the country, including in most counties in California. Until Oct. 19, it had not been confirmed in Marin. But the C.D.F.W. can’t test every bird, and Dr. Sorem said WildCare has since reported at least two dozen other suspected cases. Migratory birds that seem to be more vulnerable to the flu than in previous years have also spread it more rapidly to domestic flocks across the country. The virus has killed tens of millions of domestic chickens, turkeys and ducks, both from illness and from flocks being destroyed to prevent spread. It is walloping turkey farms nationwide and, coupled with inflation, will lead to a short supply of expensive birds this Thanksgiving. An August outbreak at Diestel Turkey Ranch in Tuolumne County killed more than 160,000 birds. Marin has no such large poultry flocks but it has many small-scale egg producers. So far, the domestic birds of the Bay Area have been spared, with the nearest large outbreaks occurring in Monterey and Sacramento Counties. But Marin farmers, and their neighbors in Sonoma County, are on edge. “Highly pathogenic avian influenza is a huge concern to our poultry industry locally, regionally and throughout the U.S.,” said Stefan Parnay, the county’s agricultural commissioner. Joy Dolcini tends 700 hens in Chileno Valley and sells the eggs through community-supported agriculture programs and at the Ferry Plaza farmers market in San Francisco. Since the bird flu outbreaks began, she has taken extra care not to allow outside birds or new people near her chickens, since humans can be carriers. But, she said, “obviously we can’t control the wildlife.” Her property, the Dolcini F Ranch, has a small pond where ducks and other birds congregate, and it sits across the road from Laguna Lake, a bird hotspot. Guidance from the state veterinarian urges poultry producers to keep their birds indoors if possible, especially if there are water bodies nearby. But Ms. Dolcini said that recommendation isn’t easy for her business, which has limited indoor space and a clientele that prizes the dark yolks and flavor of eggs from hens fed on grass and insects. “I’m really torn because I don’t have the facilities to keep my birds indoors all the time, and my customers value my eggs because the chickens are pastured,” she said. “So I really feel like I’m stuck. I’m mostly crossing my fingers.”