Mark Thompson, environmental affairs manager for mine company Montana Resources, said witnesses described the pit as like “700 acres of white birds” on 28 November.
Along with Atlantic Richfield, Montana Resources is responsible for Berkeley Pit in Butte.
Since 28 November, employees of MR and Arco had used spotlights, noise makers and other efforts to scare or “haze” the birds off the water and prevent others from landing.
The companies estimated that more than 90% of the birds had been chased off by 29 November, Thompson said.
Workers received some advance notice about the incoming flock from an off-duty Montana Resources employee about 25 miles away, who called to report there were about 25,000 geese in the air in Anaconda, Thompson said.
“I can’t underscore enough how many birds were in the Butte area that night,” Thompson said. “Numbers beyond anything we’ve ever experienced in our 21 years of monitoring by several orders of magnitude.”
The employees worked hard to save the birds, he said.
Typically, Butte sees between 2,000 and 5,000 birds all year, including spring and water migration, Thompson said.
The estimated death toll is based on drone and aircraft flights over the pit, which holds about 45bn gallons (175bn litres) of water.
Thompson said federal and state agencies were still confirming the number of dead geese. Nonetheless the company expected the total would be many times more than the 342 that died in 1995, prompting a mitigation effort that seeks to protect birds from the toxic water.
The companies would investigate to try and determine what circumstances led to “this kind of perfect storm”, with thousands of birds making a late migration and then facing a snowstorm at a time that Berkeley Pit had the only open water in the area.
University of Montana Western professor Jack Kirkley, who specializes in ornithology, told the Montana Standard that recent milder winters were not encouraging birds to head south as early and, in some cases, were causing some to stay in places where they had never stayed the winter before.
He noted there were 4m to 6m snow geese on the continent and there were some concerns that the population was too high.
MR and Arco could be fined if the EPA determines the companies were not in compliance with the bird hazing program, but Thompson said he was confident the efforts were adequate.
This article was first published by The Guardian on 07 Dec 2016.