On New Year's Day, between 4,000 and 5,000 red-winged blackbirds were found dead all over streets, driveways and roofs in Beebe, Ark.
It wasn't clear if the deaths were linked.
Meantime, scientists believe the birds in Arkansas died from blunt force trauma.
Scientists believe fireworks set off during New Year's Eve celebrations disoriented the birds, causing them to fly into homes, cars and each other.
Wildlife officials in both Arkansas and Louisiana were trying to figure out what killed the birds. They're sending carcasses to researchers at the National Wildlife Health Center in Madison, Wis. and the University of Georgia.
Also in Arkansas, about 83,000 dead and dying drum fish washed up along a 20-mile stretch of the Arkansas River, about 100 miles west of Beebe.
Wildlife officials say the fish deaths are not related to the dead birds, because mainly one species of fish was affected and it is likely they were stricken by an illness. Full test results could take up to a month.
The U.S. Geological Service's website lists about 90 mass deaths of birds and other wildlife from June through Dec. 12. Five list deaths of at least 1,000 birds and another 12 show at least 500 dead birds.
The largest was near Houston, Minn., where about 4,000 water birds died between Sept. 6 and Nov. 26 from infestations of various parasites.
The Associated Press contributed to this report.