The second unexplained mass bird death within a week has been discovered in the southern United States, this time in the state of Louisiana, officials said.

The latest incident affected some 500 birds which were discovered dead in Pointe Coupee Parish, said Olivia Watkins of the Louisiana Department of Wildlife and Fisheries.

Watkins said an investigation was underway into the cause of the deaths, which occurred just a few days after thousands of birds were discovered dead in neighboring Arkansas.

"We sent samples to a lab in Missouri and are waiting to get some results," she said.

Nancy Ledbetter at the Arkansas Game & Fish Commission said officials in that state were awaiting test results to find the cause of death of as many as 5,000 blackbirds in the small town of Beebe as well as deaths of 80,000 to 100,000 fish found floating in the Arkansas River about 160 kilometers (100 miles) away.

"We still don't believe the (fish and bird deaths) are related," she said.

As to the incident in Louisiana, she said "We don't expect that to be related either."

Officials said earlier that some type of disturbance -- possibly fireworks on New Year's Eve -- might have provoked the birds to take flight in the dark. Because blackbirds have poor night vision they may have died after bumping into houses, trees and other birds in their fright.

Arkansas officials said preliminary testing showed no signs of disease in the dead birds and that they died of "acute physical trauma."