Flu numbers tick up with fall migration, and several recent reports of dead or sick birds involve a species especially susceptible to bird flu, raising suspicion.

RIO VISTA, Calif. — Bird flu is suspected in swan deaths and illnesses reported last week in the Isleton and Rio Vista areas of the Sacramento-San Joaquin Delta, a state official said Monday.

Flu numbers tick up with fall migration, according to California Department of Fish and Wildlife (CDFW) spokeswoman Krysten Kellum. Separately, several reports the agency has received of dead or sick birds are non-native mute swans, which are especially susceptible to bird flu, she said.

Based on the time of year and the species, bird flu is suspected in the deaths, Kellum said.

She said there is a large population of swans in the area, possibly more than 1,000. CDFW received three to four swans from the Liberty Island Ecological Reserve north of Solano County’s Ryer Island and will test them this week for bird flu.

The tests take a few days to return results, according to the California agency.

Delta resident Demi Stewart said Friday she had seen three dead or sick swans in the past week and reported the sightings to CDFW.

“It’s a pretty heartbreaking thing to watch,” Stewart said.

Experts urged people and their pets to avoid direct contact with dead birds. Residents describe the same troubling pattern in ill swans: wandering in circles, looking lethargic and unaware of their surroundings before eventually dying.

Maurice Pitesky, an associate professor at the University of California, Davis, School of Veterinary Medicine, said Friday those behaviors are common symptoms of bird flu.

Pitesky said there are respiratory signs like coughing and sneezing and neurological signs like circling and a twisted neck. One common symptom of bird flu in animals is also death, he said.

“Waterfowl like wild ducks and geese are migrating into the Central Valley of California,” Pitesky said last week. “Those wild ducks and geese for the past four years now have been the main reservoir, the main source of highly pathogenic avian influenza, and unfortunately it affects them, but it also affects domestic poultry, domestic dairy and we’ve even had some human infections.”

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