Prince William County police spokesman Jonathan Perok said it happened about 2 p.m., near Aden Grocery.
Police, animal control and crews from the Virginia Department of Transportation were called to the area, where witnesses said they were shoveling dead birds off the road.
It was unclear Thursday night what type of birds they were, and what caused them to die.
Several people reported seeing large numbers of birds gathered on power lines in the area earlier in the day.
Kevin Rose, a wildlife biologist with the Virginia Department of Game and Inland Fisheries, said mass bird die-offs are usually the result of lightning or some sort of trauma. That trauma often includes birds in flight striking power lines.
"Without a few samples we can't really tell," Rose said in an email. "Unless it starts happening more, we are not concerned." - SOTT