"Most were dead," Christer Olofsson of rescue services in the southwestern town of Falkoeping said of the 50 to 100 jackdaw birds, a type of crow.
Ornithologist Anders Wirdheim said the find was surprising.
"This is unusual," he told tabloid Aftonbladet, which posted online a reader's photo of dozens of black birds littering a snow-covered road.
"They are probably jackdaws. They spend the winter in large flocks. If they are exposed to disturbances, they can become so stressed that they fly themselves to death," he said.
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Olofsson told AFP the birds were first spotted around midnight by a police patrol, and that five had been taken in for analysis.
Olov Andersson of the National Veterinary Institute told news agency TT the carcasses would be analysed and that bacterial and viral tests, including for swine flu, would be performed.
The Falkoeping incident comes after two unexplained mass bird deaths in the United States.
On Tuesday, officials in Louisiana said 500 birds were discovered dead, shortly after thousands of birds were discovered dead in neighbouring Arkansas.
Arkansas officials said preliminary testing showed no signs of disease in the dead birds and that they died of "acute physical trauma."
US officials said possibly New Year's Eve fireworks had provoked the birds to take flight in the dark, and that due to poor night vision they may have died after bumping into houses, trees and other birds in their fright.
Swedish rescue services told media on Wednesday fireworks had been heard in the area where the birds were found.
