Songbirds are mysteriously dying across the mid-Atlantic region of the USA.

Many have been discovered ill with eye swelling and discharge; some look like they're having trouble lifting their heads, indicative of head swelling. They also show neurological symptoms like tremors, disorientation, erratic flight, and lack of balance.

Over 280 sick or dead birds have been found in Indiana alone since May. Affected species include blue jays, common grackles, American robins, northern cardinals and European starlings, in states such as Ohio, Kentucky, Pennsylvania, Virginia, West Virginia, Maryland and Washington, DC.

Toxicological tests and autopsies have so far provided no conclusive answers, but have ruled out some of the usual suspects like avian influenza virus, West Nile virus and salmonella.

"As this unfolds, it is becoming a complicated puzzle," University of Pennsylvania veterinarian Lisa Murphy told The Philadelphia Inquirer. "Not one disease or disease agent jumps [out]. This does seem to be something unique and unusual."

More tests are underway.

It's like "trying to find an unknown object in a haystack", Indiana Department of Natural Resources ornithologist Allisyn Gillet said in a news briefing.

Whatever the cause of the illness, young birds appear to be especially susceptible.

Jennifer Toussaint, animal control chief in Arlington, Virginia, recently received four baby blue jays from concerned residents. A crusty ooze had sealed their eyes shut, they were lethargic and unsteady.

Each was plump, indicating "their parents had done a great job caring for them", Toussaint told Science Magazine.

Scientists and officials are now asking the public for help.

"If you encounter sick or dead birds, please contact your state or District wildlife conservation agency for further instructions and to help them track this event," the United States Geological Survey (USGS) said in an update.

The agency and other wildlife and animal experts are urging people to keep their pets contained, avoid handling birds without protection - such as a plastic bag - and to stop feeding and providing water baths for birds while this illness continues, as these activities can help spread potential pathogens.

"Clean feeders and bird baths with a 10 percent bleach solution (one part bleach mixed with nine parts water), rinse with water and allow to air dry," the USGS advises.

This is just the latest bird die-off event in the US, which is worrisome. Last year tens of thousands of songbirds, including migratory flycatchers and warblers, were found starved to death in New Mexico, Colorado, Texas, Arizona and Nebraska.

Scientists suspect this was caused by unseasonably cold weather linked to the climate crisis - most of the starved birds were insectivores.

The good news is the numbers of ill birds seem to be subsiding in the Washington area where the mysterious illness was first reported. Hopefully researchers will know more soon.

You can report sick or deceased birds here.