Over 3,000 birds were found dead in Arkansas in America on New Year's Day. Most of them were red-winged blackbirds, and no-one really knows what killed them.
As if that wasn't weird enough, 500 birds were also found dead a few days later in the neighbouring state of Louisiana. It looked like the birds had simply fallen out of the sky. Weird!
And in Sweden, around 100 jackdaws were found dead in the street. Some vets think the birds were poisoned, and tests are being carried out to find out what's going on.
So what is happening to the birds? Some people think tornados in Arkansas were to blame, and that they were hit by hailstones high in the atmosphere.
Others think the birds were so stunned by New Year fireworks they fell out of the sky. It's also possible the birds banged into objects while trying to avoid the fireworks.
There are some pretty crazy theories too: some people think the birds hit unidentified flying objects, or UFOs, spooky!
Mass bird deaths like this aren't that unusual though. These starlings dropped out of the sky in Somerset last year. It's thought they crash-landed after being chased by a predator.
And the red-winged blackbirds in Arkansas are considered a pest - so it's not uncommon for farmers to poison them. But the mystery of the latest deaths continues...