• Original Reporting
  • Sources Cited
  • Subject Specialist
Original Reporting This article contains new, firsthand information uncovered by its reporter(s). This includes directly interviewing sources and research / analysis of primary source documents.
Sources Cited As a news piece, this article cites verifiable, third-party sources which have all been thoroughly fact-checked and deemed credible by the Newsroom in accordance with the Civil Constitution.
Subject Specialist This Newsmaker has been deemed by this Newsroom as having a specialized knowledge of the subject covered in this article.
Canadian geese look for leftovers in a field at Salida, on March 29. Colorado wildlife officials are watching closely as the spring bird migration continues, hoping to avoid the large wild bird kills and mammal crossover deaths from avian flu’s 2022 epidemic. (Michael Booth, The Colorado Sun)

Four mountain lions are among the Colorado mammals felled by a crossover of the avian flu epidemic to larger animals, but state wildlife officials say the trend has slowed even as they warily eye the spring bird migration.

The bird-borne flu has also killed two bobcats, multiple skunks, two red foxes and a bear in Colorado since Jan. 1, according to records kept by the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service. The current outbreak of avian flu has killed thousands of wild birds and required the destruction of millions of egg-laying poultry since early 2022, with Colorado’s mass kills repeated in dozens of states. 

State animal health experts, though, note that the last dead mountain lion was collected Feb. 14, in Grand County. A skunk in Chaffee County was confirmed as an avian flu death April 3. But before that, 

the last mammal recorded as an avian flu crossover death was a skunk collected March 2 in Larimer County. 

A bear was discovered in Huerfano County in early January. Experts believe the mammals acquire avian flu by feeding on bird carcasses infected with the “highly pathogenic” version of the flu now circulating throughout the country. Crossover to humans in the current outbreak has been extremely rare so far. 

After mass deaths of snow geese near Eastern Plains reservoirs in the fall, no new multiple deaths of wild birds have been recorded. Raptor experts feared in late fall that the avian flu deaths of a handful of bald eagles and great horned owls could indicate a threat to species with few numbers to spare.

Arizona authorities last week announced the deaths of three endangered California condors from avian flu, and they are testing five other carcasses for the illness, according to The Associated Press. The big birds were part of a flock that spends part of the year in Grand Canyon National Park, and their cases are being handled by the National Park Service. 

Colorado, however, has not recorded any eagle deaths since late in 2022.  

Colorado residents are asked not to touch or approach bird or mammal carcasses they see in the open, or to approach animals showing signs of the disease such as confusion or odd, circling movements. Colorado Parks and Wildlife takes calls but does not test every carcass it learns about. They do continue to track any mass deaths in wild flocks. 

Wildlife officers collect carcasses when practical and lab tests help Colorado officials track the course of the outbreak across counties. Once a species has one confirmed avian flu death in one county during one season, officials consider that area affected by the epidemic and do not always test more animals from that area. 

With mass wild bird deaths and enormous poultry kills in 2022, Colorado bird experts braced for more damage once spring migration brought more birds back to the state and grouping vulnerable flocks. But spring migration is well underway, and reports are not increasing, said Colorado Parks and Wildlife Health Program Supervisor Mary Wood.

“We’ve got birds migrating March to May. So we’re right in the middle of our spring migration. It’s hard to know what we’re going to see moving forward, but it has been interesting to see a drop in cases,” Wood said.


“We have not seen any kind of really big mortality events like we did last fall. So that’s good news. We’re going to call that a win,” she said. 

There’s still a chance of migrating birds mixing with more mammals as the weather thaws. There also is the possibility of more carcass discoveries as Colorado’s historically deep snowpack melts. 

“We aren’t seeing every single case that occurs,” Wood said. “There certainly are a number of more cases that go unseen. So that makes it difficult to determine the true scale and magnitude of what’s occurring on the landscape.”

Michael Booth is the Sun’s environment writer, and co-author of the Sun’s weekly climate and health newsletter The Temperature. He is co-author with Jennifer Brown of the Colorado Book Award-winning food safety investigation “Eating Dangerously.” Booth was part of teams that won two Pulitzer Prizes for breaking news. He also writes frequently about inexplicable obsessions that include tamarisk, black-footed ferrets and tire fires. Booth also serves as the underpaid driver for four children, and plans to eventually hike every inch of Colorado.