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In the late fall in Montana, the early evening quiet is often pierced by a cacophony of honking from Snow Geese flying high overhead. Hundreds of thousands of the magnificent birds stop at lakes in the state to rest and refuel on their annual trek from breeding grounds in the far north to their overwintering sites in the south. Last week as many as , likely forced to land amid a snowstorm, settled on the toxic waters of the Berkeley Pit in Butte.

Officials are still tallying the death toll, but they estimate that thousands of geese have died from ingesting the contaminated waters of the former open-pit copper mine. Telescopes, drones, and aircraft are all being employed to count carcasses on the lake (unstable mine walls have prevented boat access for the last few years), and local residents have also found a handful of dead birds .

It appears that a combination of circumstances forced the birds to land on the poisonous pool, which is roughly a mile long and half-mile wide. Snow Geese migration has been shifting later as temperatures rise, and when they reached Montana on November 28, their usual stopover site, Freezeout Lake, was mostly frozen over, as was another landing spot near Butte that the birds often use, the reported. When a winter storm hit, the birds landed on the only open water in the area: the Berkeley Pit.

A similar scenario occurred 21 years ago, when 342 Snow Geese that landed in the pit died, the metal salts dissolved in the waters turning their snow-white feathers a rusty color. that ingesting the metal-laden brew burned the birds’ throats and caused kidney damage.

After that incident, a federally approved waterfowl mitigation plan was put in place, which involves firing rifles and employing Phoenix Wailers—devices that emit the sounds of predators and loud electronic noises—to deter birds from landing. The hazing program has been largely successful. From 2010 through 2014, for instance, a total of 17,865 birds were recorded at the pit, with only , according to the U.S. Environmental Protection Agency.       

“I can’t underscore enough how many birds were in the Butte area that night,” Mark Thompson, environmental affairs manager for mine company Montana Resources, which is responsible for the Berkeley Pit Superfund site along with the company ARCO, told the . “Numbers beyond anything we’ve ever experienced in our 21 years of monitoring by several orders of magnitude.”

The EPA, which is still investigating the case, could fine Montana Resources and Arco if it determines that the companies weren’t in compliance with the bird-hazing program.

In the meantime, the EPA’s Joe Vranka told the , Montana Resources is continuing its hazing efforts round-the-clock to keep birds out of the pit. And officials are keeping a close watch for more flocks of the clamorous birds headed toward the area.