As the East’s Songbird Epidemic Fades, the Cause Remains Unknown

The mysterious outbreak that caused widespread songbird death and sickness across the eastern and midwestern U.S., statewide feeder removal recommendations, and public concern appears to have subsided.

From late April through July, reports of disoriented and dead songbirds with swollen eyes proliferated across almost a dozen states. Most accounts were of juvenile birds, particularly American Robins, Blue Jays, Common Grackles, and European Starlings—common visitors to backyards and bird feeders. Sick birds were officially documented in Virginia, Maryland, Delaware, New Jersey, Pennsylvania, West Virginia, Kentucky, Ohio, Indiana, and the District of Columbia. Agencies in each of the states recommended taking down feeders and draining birdbaths as a precaution against social spread.

As of September 10, all states have lifted those advisories. The last agencies to do so were the Ohio and Indiana Departments of Natural Resources. The change came in response to a steep decrease in the number of reports of sick and dead birds. But even as active case numbers fall, the cause of the illness remains a mystery.

“We were wondering, you know, ‘are people going to have to just stop feeding birds long term?’” says David Curson, director of bird conservation at Audubon Mid-Atlantic. In Maryland, where Curson is based, he says the first cases emerged at the end of April, fell off through July, and had all but stopped by August. In Ohio and Indiana, initial reports of sick birds came from wildlife rehabbers in the first week of June but tapered off on a similar timeline.

At the height of the outbreak, state agencies opened online reporting systems and were overwhelmed by accounts of the illness from members of the public. “I think within the first week and half, we got 1,000 reports,” says Allisyn Gillet, the ornithologist for the Indiana Department of Natural Resources. Although many were about unrelated bird injuries and deaths, she estimates about 700 of the more than 4,300 total reports were confirmed to be part of the outbreak. In Ohio, the situation was similar. Laura Kearns, a wildlife biologist for the state’s Division of Wildlife, says she ultimately believes up to 1,000 of the thousands of total reports were about birds suffering from the same set of linked symptoms. Pennsylvania was receiving hundreds of reports daily in early July, says Andrew Di Salvo, the state’s Game Commission veterinarian. 

Now, all say accounts of sick and dead birds have reduced to a trickle in their respective states. And they attribute the few recent reports to other, more standard explanations, like window collisions. Gillet adds that new reports have shifted from Grackles, Jays, and Robins to mostly finches, which are susceptible to House Finch eye disease—a type of conjunctivitis common this time of year that can look similar to the unidentified illness.

Reports of the illness have remained low as feeders have gone back up, casting doubt on whether the condition is transmissible between birds. But there are other variables at play, says Kearns. It’s no longer breeding and nesting season, which means the vulnerable juvenile birds hit hardest by the illness have either matured or died. And it’s the beginning of migration season, so birds are on the move. Experts agree it’s hard to say why the outbreak receded when it did, and if there was any link to feeders or baths. “We're in this curious position where this wave of disease has passed us, and the people can start feeding their birds again, but we never really found out what it was,” Curson says. 

The United States Geological Survey’s National Wildlife Health Center (NWHC) has been working for months with other partner institutions to identify a diagnosis, but they don’t have a clear answer yet. Together, the group has been examining deceased birds and has released an interagency statement, last updated on July 2. In the update, they shared what’s been ruled out so far: Salmonella, Chlamydia, bird flu, Trichomonas parasites, West Nile, Newcastle disease, herpes, and other viruses haven’t been detected in any of the birds. NWHC and its partners continue to investigate, conduct tests, and to keep in contact with the affected states. 

Meanwhile biologists and state wildlife managers are divided on what they believe are the most likely possible causes and hesitant to speculate in the absence of clear evidence. Di Salvo points out that lots of potential culprits remain under investigation. “There’s all sorts of things that could be out there, and honestly we don’t know,” he says. But he is eagerly awaiting the results of time-consuming, metagenomics testing NWHC and its partners are currently conducting. Metagenomics looks at the entire bacterial community present within a cohort of sick songbirds and compares it to a control group to identify possible disease agents. 

However, for tests like metagenomics screening to offer conclusive results, they require large numbers of bird samples, and with the outbreak ending, those have become difficult to come by, says Gillet. She adds that, ultimately, having fewer samples is a good thing. “Because of the nature of the disease and how it presented itself in such an ephemeral way, we might never get to the point where we have an answer,” she says. “And that’s okay.” Her biggest hope is that the illness doesn’t reoccur next breeding season. 

To support healthy bird populations in the long term, experts recommend staying vigilant about cleaning feeders, providing only fresh food, taking down feeders if you see sick birds, minimizing pesticide use, and planting native plants. For now, though, birders can at least rest easy knowing this outbreak has passed and they can finally welcome birds back to their feeders.