ST. GEORGE, Utah • Thousands of migratory birds were killed or injured after apparently mistaking a Walmart parking lot, football fields and other snow-covered areas of southern Utah for bodies of water and plummeting to the ground in what one state wildlife expert called the worst mass bird crash she'd ever seen.

Crews went to work cleaning up the dead birds and rescuing the injured survivors after the creatures crash-landed in the St. George area Monday night.

By midday Wednesday, volunteers had helped rescue more than 3,000 birds, releasing them into a nearby pond. There's no count on how many died, although officials estimate it's upward of 1,500.

"They're just everywhere," said Teresa Griffin, wildlife program manager for the Utah Division of Wildlife Resource's southern region. "It's been nonstop. All our employees are driving around picking them up, and we've got so many people coming to our office and dropping them off."

No human injuries or property damage have been reported.

Officials say stormy conditions probably confused the flock of eared grebes, a ducklike aquatic bird probably making its way to the Mexican coast for the winter.

The birds plunged into a Cedar City Walmart parking lot, football fields, highways and over miles of property that had been blanketed by about 3 inches of gleaming snow.

"The storm clouds over the top of the city lights made it look like a nice, flat body of water. All the conditions were right," Griffin told the Spectrum newspaper in St. George. "So the birds landed to rest, but ended up slamming into the pavement."

Kevin McGowan, who studies birds at the Cornell Lab of Ornithology in Ithaca, N.Y., said grebes rely on starlight to navigate during their nighttime migration.

"Before there were (artificial lights), the sky was always paler than the ground," he said. "When all of a sudden there's light all over the place, they don't know which way is up anymore."