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Great horned owls are kept in the hospital area of the Rocky Mountain Raptor Program in Fort Collins. The program rehabilitates around 300 birds a year, 78% being treatable cases that can be returned to the wild. (Olivia Sun, The Colorado Sun via Report for America)

Bird flu has killed at least four bald eagles in Colorado and threatens hundreds of other rare raptors and wild birds, as Colorado agriculture and wildlife officials struggle with an outbreak they call the worst in national or state history

Colorado’s current bout with avian influenza has already forced farmers to mass slaughter more than 4 million chickens from egg operations, and now wildlife officials worry the highly infectious disease may have contributed to a 15% to 20% drop in successful eagle nesting this year. 

Bird experts fear for dozens of year-round bald eagles in the northeastern Colorado corridor of reservoirs and wetlands along the South Platte River, after a mass snow goose die-off killed more than 2,000 birds near Julesburg/Jumbo Reservoir. Waterfowl pick up the disease from saliva, mucous and feces of other birds. Raptors and other carrion birds also spread the disease feeding on carcasses undiscovered by wildlife officials or neighbors. 

Raptor centers like Rocky Mountain Raptor Program in Fort Collins are taking in a steady stream of great horned owls and red-tailed hawks dying from the fast-acting virus. Staff are donning Tyvek protective suits and other PPE in an effort that is more hospice than rescue. 

State wildlife officials assume there are more dead bald eagles than the four confirmed so far in 2022 by tests of carcasses. With fewer than 300 nesting pairs of bald eagles making their homes along Colorado’s waterways, losses from any new threat to the more rare raptors quickly raise warning levels. 

“As someone who really loves birds, and that’s why we chose this profession, I am hurting,” said Zach Hutchinson, a naturalist and community science coordinator for Audubon Rockies in Colorado, Wyoming and Utah. “And I am really nervous as to what this could become.” 

A golden eagle is seen through a spotting scope in Loveland near the only known golden eagle tree nest along the northern Front Range on Friday, Jan. 7, 2022. (Valerie Mosley, Special to the Colorado Sun)

With thousands of waterfowl and other migratory birds on the move across Colorado on their way to comfortable winter ground, infections could increase even as farmers take desperate measures to stop the virus in domestic flocks. The state’s updated listing of avian flu deaths now includes two snow goose die-offs of more than 2,000 birds in late November in Morgan and Logan counties, and another report Thursday of more than 600 snow geese in Prowers County.

“We are definitely concerned about the wild birds. We know those migratory patterns are really what helps spread the virus,” said Olga Robak, communications director for the Colorado Department of Agriculture. The handful of large egg producers now trying to replace slaughtered flocks still face avian flu threats from wild birds remaining near or crossing over their farms. 

“This has been the largest outbreak in Colorado and the nation, ever,” Robak said. 

In past outbreaks, wild birds appeared to be carriers of the influenza but did not usually suffer from symptoms themselves, Robak said. “This year, the outbreak has affected many more wild birds, and even some mammals, like foxes and skunks, than in years past. Including raptors.” 

While all bird losses are upsetting to rescue crews, Colorado has larger populations of owls and hawks to absorb losses, said Mike Tincher, rehabilitation coordinator at Rocky Mountain Raptor Program. How raptors with far fewer numbers emerge from the current outbreak is still an open question, he added. 

“A lot of bald eagles come from up north, and they spend the winters here because we have relatively mild conditions,” Tincher said. “And they follow the food supply, which happens to be lots and lots of geese. So, with them being opportunistic scavengers, that’s a big concern.”

The wildlife and agriculture agencies want the public to call in reports of carcasses or live birds behaving erratically, but they warn against touching any remains. Signs of sickness include swimming in circles or erratic patterns, lethargy and inability to fly. Humans have only rarely contracted avian influenza, but the virus is easily spread on shoes or clothing to other birds and vulnerable animals. 

The state’s list of confirmed avian influenza deaths in wild birds includes eagles, hawks, owls, teals, mallards and wood ducks, and turkey vultures. Song birds and backyard birds do not appear to be heavily impacted so far, perhaps because they do not mix as much with the waterfowl that are the primary spreaders. 

State and federal officials are still working with the large egg operations to clean up and decontaminate after losing millions of chickens to quarantine efforts. Colorado estimates 4.7 million chickens were destroyed by last week. The chickens are usually killed en masse by locking down the large holding sheds and either smothering them with carbon dioxide, firefighting foam, or by stopping ventilation and allowing the resulting heat rise to kill the flock. Authorities want the dead birds buried in excavations on site rather than further the spread. 

Destroying that many living creatures is the most devastating part of the outbreak so far, Robak said. 

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“It is a very hard and emotional thing for the people who are responding to this emergency, but especially to the people who own these operations,” she said. “This is their daily life. This is what they do.” 

Researchers hope to learn more about the behavior of the virus and bird reactions as the outbreak rages on. State wildlife officials had been capturing live bald eagles and tagging them for tracking as part of another study, and that tracking is proving valuable in watching avian flu survival. 

The female in one nesting pair watched by the state is one of the bald eagle flu deaths, said Colorado Parks and Wildlife avian researcher Reesa Conrey. Researchers have been watching the male partner’s movements since then.

“He’s flown over his nest a couple of times, but he’s flown all over the state and down into New Mexico and back. So he’s definitely alive and doing OK if he’s making those kinds of long-distance movements,” Conrey said. Researchers want to know more about why some birds appear to be immune even when in close contact with diseased birds. Studies are made more challenging by the speed of the virus — many birds die within two or three days of showing symptoms. 

With just a few hundred eagles across Colorado, wildlife researchers are avid followers of their life stories. 

“I don’t know whether he will find a new mate and go back to that nest and use it in 2023,” Conrey said. “I think that remains to be seen.”

Michael Booth

Michael Booth is a Colorado Sun reporter covering health, health policy and the environment. Email: booth@coloradosun.com Twitter: @MBoothDenver