Some 10,000 migrating snow geese have died after landing in a lake laden with toxic heavy metals and sulphuric acid.
Maintaining the lake, which has formed in an old open pit mine in Butte, Montana, is the responsibility of Montana Resources and Atlantic Richfield.
The companies participate in a Government-ordered 'hazing' scheme to scare geese with loud noises, instituted after a much smaller die-off in 1995.
Montana Resources and the Environmental Protection Agency (EPA) are using drones and aircraft to accurately count the number of dead birds, the Billings Gazette reported.
Mark Thompson, the Berkeley Pit's environmental affairs manager, said several dozen live geese are still in the pit, swimming in the toxic water, after landing as part of a flock some 25,000 strong on November 28.
He told AP: "I can't underscore enough how many birds were in the Butte area that night.
"Numbers beyond anything we've ever experienced in our 21 years of monitoring by several orders of magnitude."
Workers did "incredible things to save a lot of birds and they really put their heart and soul behind it," he added.
Last year about 2,000 snow geese fell out of the sky in Idaho, apparently killed by avian cholera.
The species was considered endangered in the early 20th century but recovered after hunting was banned.
More than 300 died in 1995 after drinking contaminated Berkeley Pit water and examinations showed heavy metals in their kidneys.
The thousands of geese that perished in the most recent incident had flown past their usual landing spot - called Freezeout Lake - because it was mostly frozen, the Montana Department of Fish, Wildlife and Parks told the Gazette.
Montana Resources has collected 20 birds for the department to analyse, and mine officials said they had asked federal authorities whether they expected further large migrations over the area.
The EPA could fine Montana Resources after investigating whether it was sticking to the hazing scheme.
The programme had apparently been successful in its first five years, from 1996 to 2001, with 22,000 birds observed over the Berkeley Pit but just 75 deaths.
Montana Resources and BP, Atlantic Richfield's parent company, have been contacted for comment.
