In Minnesota, clues behind an “absolutely unprecedented” outbreak of bird flu in turkey flocks point to wild ducks on a spring break during northern migration.

One of the most-recent victims: 310,000 turkeys in Meeker County.

Thirteen cases of highly pathogenic avian influenza in commercial flocks have been confirmed since the beginning of March in the state, the top turkey producer in the United States. As agriculture authorities investigate, signs point to abundant water basins as the source of the trouble. Nicknamed the Land of 10,000 Lakes, Minnesota attracts scads of geese, ducks and other birds.

Migratory waterfowl, which can be “natural reservoirs” for some strains of the disease, are believed to be spreading the virus, Beth Thompson, assistant director of the St. Paul- based Minnesota Board of Animal Health, said in a telephone interview on Wednesday.

A team of epidemiologists is visiting infected flocks in the state to investigate how the virus ends up in barns, she said.

“Between the time of year and the number of turkey barns in the state, it’s all coming together,” Thompson said. “We believe that’s how the virus, at this point in time, is getting here.”

Almost 47 million young turkeys were slaughtered in Minnesota last year, a fifth of the U.S. total, USDA data show. Commercial flocks in the state are primarily raised indoors, Thompson said.

The flu was discovered in thirteen flocks with more than half a million birds in nine Minnesota counties, USDA reports shows. That’s more than any other state, according to the Animal and Plant Health Inspection Service, part of the nation’s Department of Agriculture. The premises are quarantined, and the turkeys are “depopulated” to prevent the spread of the virus, the agency says.

Gobblers raised for sale in supermarkets have also been infected in Missouri, Arkansas and South Dakota. Since the outbreak began in the western U.S. late last year, just one commercial chicken flock has been infected, in Kings County, California, according to the USDA.

Turkeys are “a little bit more susceptible” to the flu than chickens, meaning a smaller amount of the virus can infect the birds, David Swayne, the director of the USDA’s Southeast Poultry Research Laboratory in Athens, Georgia, said in a telephone interview on Wednesday. The lab has examined the first strains of the virus that were detected in the U.S., he said.

Turkeys aren’t “a lot more susceptible,” though, Swayne said. “It doesn’t explain everything that’s happening in the field. We will be doing additional studies looking at the newer viruses to find out whether they have changed in their ability to infect and transmit.”

The raft of Minnesota turkey cases is “absolutely unprecedented,” Carol Cardona, the Pomeroy Chair in Avian Health at the University of Minnesota, said in a telephone interview on Monday. Before this year, the only highly pathogenic case in U.S. commercial poultry occurred in Texas in 2004, according to data from the the Centers for Disease Control and Prevention that goes back to 1997.

Viruses such as avian influenza thrive in cool, damp weather, Thompson said. The risk to humans from strains detected in the U.S. is low, the USDA has said, citing the CDC.

“Once it starts warming up and drying out in Minnesota, I think that’s going to play a role,” she said. “The virus will be dying out then. It won’t be staying in the environment as long.”