Investigators in Sweden are trying to find out what caused the death of around 50 birds found in a street in the city of Falkoping.
Fireworks had been set off near the scene earlier, said vet Robert ter Horst.
Cold weather, difficulties finding food and possible shock from the fireworks could be responsible, leading to the birds dying from the stress or being run over by vehicles.
Five of the dead jackdaws were being tested.
Mass bird deaths are not uncommon.
In the US, New Year's Eve fireworks were blamed in Arkansas for the deaths of thousands of blackbirds, and a few days later power lines were blamed for killing about 450 birds in Louisiana.