Preliminary autopsies on 17 of the up to 5,000 blackbirds that fell on this town indicate they died of blunt trauma to their organs, the state's top veterinarian told NBC News on Monday.
Their stomachs were empty, which rules out poison, Dr. George Badley said, and they died in midair, not on impact with the ground.
That evidence, and the fact that the red-winged blackbirds fly in close flocks, suggests they suffered some massive midair collision, he added. That lends weight to theories that they were startled by something.
Earlier Monday, the estimated number of dead birds was raised to between 4,000 and 5,000, up sharply from the initial estimate of 1,000.
Keith Stephens, a spokesman for the Arkansas Game and Fish Commission, provided the new numbers.
Residents of the small town of Beebe awoke Saturday to find thousands of dead blackbirds littering a 1.5-square-mile area. The birds inexplicably dropped dead, landing on homes, cars and lawns.
Violent weather rumbled over much of the state Friday, including a tornado that killed three people in Cincinnati, Ark. Lightning could have killed the birds directly or startled them to the point that they became confused. Hail also has been known to knock birds from the sky.
The director of Cornell University's ornithology lab in Ithaca, N.Y., said the most likely suspect is violent weather. It's probable that thousands of birds were asleep, roosting in a single tree, when a "washing machine-type thunderstorm" sucked them up into the air, disoriented them, and even fatally soaked and chilled them.
"Bad weather can occasionally catch flocks off guard, blow them off a roost, and they get hurled up suddenly into this thundercloud," lab director John Fitzpatrick said.
Rough weather had hit the state earlier Friday, but the worst of it was already well east of Beebe by the time the birds started falling, said Chris Buonanno, a forecaster with the National Weather Service in North Little Rock.
If weather was the cause, the birds could have died in several ways, Fitzpatrick said. They could easily become disoriented — with no lights to tell them up and down — and smack into the ground. Or they could have died from exposure.
The birds' feathers keep them at a toasty 103 degrees, but "once that coat gets unnaturally wet, it's only a matter of minutes before they're done for," Fitzpatrick said.
Lightning or hail are also possibilities.
Karen Rowe, an ornithologist for the state commission, noted that in 2001 lightning killed about 20 mallards at Hot Springs, and a flock of dead pelicans was found in the woods about 10 years ago. Lab tests showed that they, too, had been hit by lightning.
Moreover, in 1973 hail knocked birds from the sky at Stuttgart, Ark. Some of the birds were caught in a violent storm's updrafts and became encased in ice before falling from the sky.
Rowe noted that birds of prey and other animals, including dogs and cats, ate several of the dead blackbirds and suffered no ill effects.
"Every dog and cat in the neighborhood that night was able to get a fresh snack that night," Rowe said.
Mike Robertson, the mayor in Beebe, said the last dead bird was removed about 11 a.m. Sunday in the town about 40 miles northeast of Little Rock. A dozen workers hired by the city to do the cleanup wore environmental-protection suits for the task.
Robertson said the workers wore the suits as a matter of routine and not out of fear that the birds might be contaminated.
"It started at 7 a.m., picking up birds on the street, in the yards, been run over. It's just a mess," Beebe Street Department supervisor Milton McCullar told WISC-TV.
Several hundred thousand red-winged blackbirds have used a wooded area in the town as a roost for the past several years.
Robertson and other officials went to the roost area over the weekend and found no dead birds on the ground.
"That pretty much rules out an illness" or poisoning, the mayor said.
But some residents voiced concerns.
"I've been to Iraq and back and not seen nothing like this," Beebe resident Jeff Drennan told local Fox16 News on Sunday.
"You know my kids are out here playing and you don't know, is it safe?" he added. "They're walking around with chemical suits picking them up with gas masks and everything."
The birds will not be missed. Large blackbird roosts like one at Beebe can have thousands of birds that leave ankle- to knee-deep piles of droppings in places.
Nearly a decade ago, state wildlife officials fired blanks from shotguns and cannons to move a roost of thousands of blackbirds from Beebe, but in recent years many of the migratory birds returned.
Red-winged blackbirds are the among North America's most abundant birds, with somewhere between 100 million and 200 million in the U.S., Fitzpatrick said.
Near Ozark, a town 125 miles from Beebe, up to 100,000 fish were found dead along the Arkansas River, leading to speculation of a connection.
David Lyons, the head of a local chapter of the Sierra Club, told msnbc.com that he was "waiting for the results of the pathology and toxicology tests before I make any judgments about the bird and fish kills.
"So far, the evidence does not suggest that pollution contributed to either the bird or fish kill," he added. "If the test results indicate that contaminants were responsible, then local environmental groups will likely have several questions and concerns about the two events."