Thousands of migratory birds died on impact after apparently mistaking a Wal-Mart car park in the US and other areas of southern Utah for bodies of water and plummeting to the ground in what one wildlife expert has called the worst downing she's ever seen. Crews went to work cleaning up the dead birds and rescuing the survivors after the creatures crash-landed in the St George area on Monday night. By Tuesday evening, volunteers had rescued more than 2000 birds, releasing them into nearby bodies of water. "They're just everywhere," said Teresa Griffin, wildlife program manager for the Utah Department of Wildlife Resource's southern region. "It's been non-stop. All our employees are driving around picking them up, and we've got so many people coming to our office and dropping them off." Officials say stormy conditions probably confused the flock of grebes, a duck-like aquatic bird likely making its way to Mexico for the winter. The birds tried to land in a Cedar City Wal-Mart car park and elsewhere. "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." No human injuries or property damage have been reported. Griffin noted most downings are localised "but this was very widespread". "I've been here 15 years and this was the worst downing I've seen," she told the newspaper. Officials said they were continuing the rescue effort that started on Tuesday afternoon and included an enthusiastic group of volunteers. The surviving grebes were released into bodies of water in southern Utah's Washington County, including a pond near Hurricane. "If we can put them on a body of water that's not frozen over, they'll have a better chance of survival," said Lynn Chamberlain, a wildlife department spokesman.