At least 50 of the black birds were found dead near the intersection of King Street and Dufferin Avenue on Wednesday, baffling many people who live and work in the area.
Preliminary test results have eliminated the West Nile virus, avian influenza and Newcastle disease as causes of death, a provincial government spokesperson told CBC News on Monday.
Tests on the deceased birds are still in progress, the spokesperson said.
Witnesses reported seeing hundreds of grackles flocking in a "bizarre manner" early Wednesday morning atop vehicles, in the trees and near businesses.
Dozens of the birds were then seen dropping from the sky and falling out of the trees later that morning.
Manitoba Conservation collected the dead grackles while the Winnipeg Humane Society picked up 11 birds that were still alive.
The 11 birds, which could not stand or fly, were to be euthanized and sent to a pathology laboratory for necropsies.