BUTTE — Some of the more than 1,000 snow geese that landed to rest on the highly acidic water in a former open-pit copper mine in Butte have died.

Mark Thompson, manager of environmental affairs for Montana Resources, tells The Montana Standard the dead birds were spotted at the Berkeley Pit early Thursday morning. The agency no longer puts boats in the water out of concern that an unstable pit wall could collapse, so it’s unclear how many birds have died.

The birds arrived Monday and some flew away, but hundreds remain at the pit. Environmental officials have been shooting rifles and using other noise tactics to get them to leave.

Thompson says as many as 10,000 birds landed Monday night, but about 9,000 took flight Tuesday morning.

About 340 migrating geese died at the pit in 1995.