A 'mysterious illness' is plaguing birds and baffling scientists across parts of the U.S.

A "mysterious illness" is plaguing "hundreds" of birds across the southern and midwestern U.S., leaving them with "crusty eyes, swollen faces, and the inability to fly," say The Guardian and HuffPost. Reports of such afflictions began in the spring, and scientists are still unsure as to their cause.

Other potential symptoms include seizures or loss of balance — birds may even "just sit still, often kind of shaking," said Kate Slankard, a biologist of the Kentucky Department of Fish and Wildlife Resources.

In Kentucky, blue jays, common grackles, and European starlings are among the species most affected, while the Wildlife Center in Ohio said on Facebook it had been "admitting songbirds with eye issues." Indiana officials made sure to test afflicted birds for avian influenza and west Nile virus, but both samples came back negative, reports the Guardian. States like Maryland, Virginia, West Virginia, and Washington, D.C. have also reportedly seen cases, writes HuffPost.

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The U.S. Geological Survey has recommended people refrain from feeding the avian creatures "until this mortality event has concluded," as birds congregating around feeders and baths can transmit disease to one another, the Guardian writes. The USGS also recommends, for the time being, cleaning feeders and baths with a 10 percent bleach solution, and avoiding the handling of birds.

"This is truly scary," said Jim Monsma of Washington, D.C. wildlife rescue group City Wildlife to DCist last week. "And it's just every day more and more birds."

Read more at The Guardian.