The bird flu has struck an egg-laying operation in Iowa owned by prominent Minnesota businessman Glen Taylor, and it could be the single largest casualty yet of the lethal virus sweeping the Upper Midwest poultry industry.

The Iowa Department of Agriculture said Thursday that there was an initial positive test at an egg-laying operation in northwestern Iowa with an estimated 5.5 million birds. Rembrandt Enterprises, one of the nation's largest egg producers, confirmed the operation is its Rembrandt, Iowa, facility.

Taylor owns Rembrandt, as well as the Minnesota Timberwolves and the Star Tribune.

Also on Thursday, wildlife regulators announced that a hawk in western Minnesota is the state's first wild bird to test positive for the bird flu, adding more mystery to the virus' spread.

And three more Minnesota turkey farms were reported stricken by the flu, bringing the number of afflicted farms to 70 and the number of bird deaths to almost 4 million — about 8 percent of the state's annual turkey production.

In Iowa, even before the Rembrandt disclosure, the H5N2 bird flu this week was confirmed at four other egg-laying farms. Without the Rembrandt farm, the U.S. Department of Agriculture placed the number of doomed chickens in that state at 10 million, about 17 percent of the state's layer hens. Iowa is the nation's largest source of eggs; Minnesota is the biggest U.S. turkey producer.

At the Rembrandt facility, the avian flu has been found in one barn containing about 250,000 hens, said Jonathan Spurway, the company's vice president of marketing and optimization. "It is isolated in that barn."

The company is talking with the USDA about the fate of the rest of the flock. So far, all "flock plan" agreements with the USDA have called for the destruction of every bird on a stricken farm. If all affected hens on the Rembrandt property are killed, it would be the largest single poultry loss since the bird flu surfaced in North America last winter and would bring the percentage of Iowa layer hens lost to 25 percent.

Birds killed by the disease are financial losses borne by the farm's owner. The USDA pays growers an indemnity for poultry that are euthanized. The indemnity, the USDA acknowledges, does not make the grower whole.

Rembrandt Enterprises was the nation's third-largest egg company at the end of 2014, with 14.5 million hens, according to Egg Industry, a trade magazine. Rembrandt said its egg operations in Renville and Thompson, Iowa, have not been affected by the flu.

The ultimate source of the bird flu is believed to be wild waterfowl, yet exactly how it's creeping into enclosed barns — and for that matter the recently infected Minnesota hawk — is puzzling animal health experts.

"We are really in research mode," said Carol Cardona, a veterinary bioscience professor at the University of Minnesota. "There's a whole bunch of stuff we don't know."

While deadly to poultry, the bird flu is a low risk for human health, and there are no U.S. reports of people getting sick from it.

The Minnesota Department of Natural Resources collected the dead Cooper's hawk in Yellow Medicine County as part of its effort to track and understand the spread of H5N2 bird flu. Waterfowl don't themselves get sick or die from the flu, but raptors — birds of prey — are thought to die once infected.

A homeowner near St. Leo reported that on April 14, the Cooper's hawk flew into his home's deck and died. "The immediate cause of death was running into a window," said Patrick Redig, a University of Minnesota veterinary professor and co-founder of the school's Raptor Center. Later testing showed that the bird had been exposed to H5N2.

The hawk discovery doesn't indicate the virus in wild birds is the direct cause of the bird flu, said Lou Cornicelli, DNR Wildlife Research manager. Yellow Medicine County doesn't have any infected poultry farms, but nearby Lyon County does.

The DNR said it does not know of any recent raptor die-offs.

The Raptor Center has been testing live eagles, owls, hawks and peregrine falcons since January, not long after the highly pathogenic bird flu surfaced in North America. After several dozen tests, "we have yet to encounter any influenza" in those birds, Redig said.

"Time will tell if this is an anomaly or indicative of something larger," he said.

Raptors would likely get the bird flu from eating infected waterfowl. However, a Cooper's hawk, while it eats small birds, does not eat waterfowl, Cornicelli said. "That's where we get more questions."

The DNR has been sampling waterfowl feces in Minnesota over the past month, looking for the H5N2 virus, but finding nothing.

The agency has collected 2,749 samples, nearing its goal of 3,000. "That's a lot of samples," Cornicelli said. More than 2,200 results have been negative, and over 500 tests are pending.

Across the country, no environmental fecal samples from waterfowl have tested positive for highly pathogenic avian flu, according to the USDA.

The flu has been detected in roughly 50 waterfowl — either dead or alive — in western states, notably Oregon and Washington, according to the USDA. Those birds, which travel the migratory bird route known as the Pacific flyway, particularly include mallard ducks. Three geese in Missouri were found to have the virus in March, while a goose and duck tested positive this week in Kentucky.

Missouri and Kentucky are on the Mississippi flyway, the route birds travel to and from the south through the Upper Midwest.

Mike Hughlett • 612-673-7003