US scientists believe fireworks may have caused thousands of birds to fall from the sky over an Arkansas town on New Year's Eve.
Karen Rowe, of the Arkansas Game and Fish Commission, said the red-winged blackbirds probably flew low to avoid explosions and collided with objects.
However, she stopped short of declaring the mystery solved, saying further tests on the dead birds are planned.
Officials say more than 3,000 birds fell over the city of Beebe.
The few that survived their fall stumbled around like drunken revellers, witnesses said.
Birds were "littering the streets, the yards, the driveways, everywhere," said Robby King, a county wildlife officer.
"It was hard to drive down the street in some places without running over them."
Initial laboratory reports said the birds had died from trauma, the AGFC said.
Residents reported hearing loud fireworks just before the birds started raining from the sky.
Severe weather over Arkansas could also be the cause of the mystery deaths.
"They started going crazy, flying into one another," said AGFC spokesman Keith Stephens.
The birds also hit homes, cars, trees and other objects, and some could have flown hard into the ground.
"The blackbirds were flying at rooftop level instead of treetop level" to avoid explosions above, said Ms Rowe, an ornithologist.
"Blackbirds have poor eyesight, and they started colliding with things."
Beebe police chief Capt Eddie Cullum said they were inundated with calls from residents who saw the birds fall.
Poisoning has been ruled out after several cats and dogs that ate the dead birds suffered no ill effects, he added.
However, another theory is that a violent thunderstorm could have disoriented a roost of blackbirds.
Tornadoes swept through Arkansas and neighbouring states on 31 December, killing seven people.
City authorities have hired a specialist waste disposal firm to collect the dead birds from gardens and rooftops, and remove them.