Veterinarian Robert ter Horst says the cause of jackdaws' deaths was unclear but that fireworks were set off near the scene Tuesday night.
The birds were found dead on Wednesday.
Ter Horst says cold weather, difficulties finding food and possible shock from the fireworks could be responsible, leading to the stressed birds either dying from the stress or being run over by vehicles.
Five of the dead jackdaws found in the city of Falkoping were being tested.
Mass bird deaths aren't uncommon.
Still, a recent string of such events has raised public interest, and even mostly-joking suggestions that the occurrences are signs of the .
Around 5,000 .
Some 500 birds littered a quarter-mile stretch of highway in .
Meanwhile, large fish kills were reported in Arkansas - near the site of the bird deaths - and Brazil.