Whether it's a single death or a mass die-off, experts from Texas A&M AgriLife and other agencies say almost all bird mortality in Texas and elsewhere is due to natural – or at least explicable – causes. Each year in the U.S., hundreds of millions of birds die from a variety of causes, according to Dr. Thomas Lacher, head of the wildlife and fisheries sciences department at Texas A&M University in College Station.

"The larger bird die-offs we see in Texas this time of year are not all that unusual given the kind of weather related to the season, storm fronts and mass roosts of birds, especially blackbirds," Lacher said. “Mass bird die-offs in the hundreds happen all the time, but we seldom see evidence of them."

However, Texas AgriLife Extension Service wildlife specialist Dr. Jim Gallagher, who works at the Texas AgriLife Research and Extension Center in Uvalde, is someone who has witnessed a sudden, unexpected mass bird die-off.

"Many years ago, while I was living in upstate New York, I saw dozens of geese crash to the ground when they were suddenly caught in a freezing rain," he said. "The weight of accumulated ice on them made it impossible to sustain flight."

In recent years, Texas has had its share of unusual, even “bizarre” weather, Gallagher noted, and birds are especially vulnerable to the vagaries of sudden cold, unpredictable winds, hail and lightening.

"If you've ever been on a heavy commercial aircraft that the wind suddenly moved up or down 1,500 feet or more in a matter of seconds, think what that kind of force could do to a bird weighing only ounces," he said. "In an updraft, masses of birds can also accumulate ice on their wings and bodies at higher altitudes. And in a sudden downdraft, especially one associated with something like a micro-burst, a mass of them can be tossed to the ground."

Gallagher said birds thrust thousands of feet upwards by a sudden updraft also are subjected to physical stress similar to that of a diver trying to resurface too quickly.

"Basically, dissolved gases in their blood suddenly start boiling out and they get the avian equivalent of the bends," he said. "A bird flying along at 1,000 feet and suddenly being thrust upwards to 20,000 feet will be subject to the same physical effects as a diver coming up to the surface too quickly — if the rise is rapid enough."

Disease and parasites may also be factors in some mass bird deaths, according to experts at the Texas Veterinary Medical and Diagnostic Laboratory in College Station.

Necropsies done by the lab on birds from a 60-plus bird die-off Jan. 8, 2007 in Austin revealed they were "heavily parasitized." But the unusually cold weather the night before was given as a "principal factor" in this die-off, which led to the temporary closure of several downtown streets by state health officials until the incident was dismissed as a public health threat.

"There has been evidence that a few wild-bird deaths in Texas over the past several years have been associated with West Nile virus," said Dr. Randy Moore, resident director of the diagnostic lab’s poultry laboratory in Center. "We historically have seen instances of West Nile, which is predominantly carried by mosquitoes, affecting birds here in Texas, but the number of birds is very small. And currently there is no evidence that avian influenza (bird flu) or other avian viruses have been associated with mass die-offs in wild bird species in the United States."

Moore said mortality from parasites or disease is more often associated with individual or small groups of birds and is usually a "contributing factor" in these situations as opposed to a singular cause of death.