Just days after thousands of blackbirds dropped out of the sky over a small town in Arkansas, another 500 birds fell to their death over a Louisiana highway.

Carcasses from both groups have been sent to researchers at the National Wildlife Health Center in Madison, Wisconsin, and the University of Georgia , but at the time being there is no clear link.

Such massive wildlife kills are not uncommon.

The results of tests on the dead birds in Arkansas are in, but investigators say the mystery may never be completely solved. However, scientists say it's probable New Year's Eve fireworks played a role, scaring the birds into a frenzy, causing them to run into each other.

George Badley, the state veterinarian with Arkansas' Livestock and Poultry Commission, says tests on a handful of the thousands of dead birds all showed internal injuries.

"The 17 birds we did a necropsy on had internal bleeding that looks like blunt force trauma -- like they ran into something, would be our best guess," Dr. George Badley told CTV's Canada AM Tuesday morning.

Tests have, so far, ruled out poison and lightning as the cause of the birds' mass death.

"They all had internal bleeding and no signs of disease, but our pathologist is continuing to make sure that they are no viruses, germs or poisons in the birds," Badley added.

Late Monday, the Chief of the Wildlife Management division at the Arkansas Game and Fish Commission which is heading up the probe, suggested fireworks may be to blame.

"They get some kind of loud noise that would spook them . . . and they don't generally fly at nighttime and they start crashing into each other," David Goad told CTV News Channel, explaining that locals describe the birds as flying low at the time.

Goad said the birds likely suffered severe trauma in those mid-air collisions, subsequently forming blood clots that helped kill them.

However, he said it was possible the birds were so scared by the fireworks that they just wore "their muscles out."

Between 11:30 p.m. and midnight on New Year's Eve, thousands of birds plummeted to the ground over an area about 1.5 kilometres long and 800 metres wide in Beebe, Ark.

When it was over, the community of about 5,000 people northeast of Little Rock were left to pick up the carcasses. Most were red-winged blackbirds, though grackles and starlings were also among the dead.

Just one day earlier, tens of thousands of dead and dying fish washed up along a 40-kilometre stretch of the Arkanasa River near the city of Ozark about 200 kilometres west of Beebe.

While the timing and proximity has fuelled a variety of conspiracy theories linking the events, Badley says officials don't believe the mass deaths are connected.

"Everyone here in Arkansas believes that these two events are completely unrelated," Badley told Canada AM.

Because only drum fish washed up along the Arkansas River, experts have suggested the die-off was likely due to disease, rather than contamination.

It could take up to one month for the results of tests on the dead fish.