The early morning chorus of chirps and tweets has taken on a morose note as a mysterious illness is harming Virginia songbirds.

Since late May, people have been reporting sick and dying birds afflicted with swollen, crusty eyes and neurological symptoms — such as balancing issues and tremors. What began in a handful of states has now expanded to include Washington D.C., Maryland, Virginia, West Virginia, Delaware, New Jersey, Pennsylvania, Ohio and Indiana.

According to a statement released by the Virginia Department of Wildlife Resources, cases in Virginia so far have been confined to the state’s northern and northwestern regions. Diseases can spread where birds congregate, such as bird feeders and birdbaths, and wildlife experts are asking residents in this area to remove their feeders and keep birdbaths clean as a precaution.

“Experts are still trying to determine what’s causing the bird deaths,” said Stefanie Taillon, assistant director of governmental relations for Virginia Farm Bureau Federation. “As with any unusual human-wildlife encounter, it’s important to report sightings to the appropriate officials.”

The DWR says between May 23 and June 30, it received more than 1,400 reports of sick or dying birds in the northern and northwest areas, 450 of which involved birds with eye issues and/or neurological signs. Many species of birds have been affected, but most instances are among fledgling common grackles, blue jays, European starlings and American robins.

Officials say there are no reports of the mysterious illness causing health issues in poultry, livestock or humans.

The specific illness is still unknown, and “multiple bacterial, viral and parasitic agents have not been detected in the bird samples tested,” said Sergio Harding, bird conservation biologist for the DWR. “We continue to work with diagnostic laboratories to investigate the cause of this event.”

Among the potential causes that have been ruled out are: salmonella, chlamydia and viruses including avian influenza, West Nile, other flaviviruses, Newcastle disease virus and other paramyxoviruses, herpesviruses and poxviruses; and Trichomonas parasites.