LOS ANGELES — Scientists still don’t know what’s causing flocks of birds to drop from the sky in the South, even as several hundred more fell dead onto a Louisiana highway.

The puzzling phenomenon started on New Year’s Eve when thousands of blackbirds were found dead in central Arkansas. Townsfolk spent their holiday weekend removing the remains of between 4,000 and 5,000 dead red-winged blackbirds. Scientists have descended on the town, trying to find a cause for the mass die-off.

In the latest incident, roughly 500 birds were discovered scattered on a rural road in Louisiana’s Pointe Coupee Parish, 300 miles south of the Arkansas site. Officials there are stumped as to what caused the birds to plunge to their death.

Wildlife officials in both states were sending carcasses to researchers at the National Wildlife Health Center in Madison, Wis., and the University of Georgia. No one is yet connecting the two mass deaths, but the Audubon Society is monitoring the situation.

“Mass bird die-offs can be caused by starvation, storms, disease, pesticides, collisions with manmade structures or human disturbance,” says Greg Butcher, Audubon’s director of bird conservation. “Scientists are still investigating what happened to the birds in Louisiana and Arkansas, but initial findings indicate that these are isolated incidents that were probably caused by disturbance and disorientation.”