BILLINGS, Mont. (AP) â¿¿ Oil company SM Energy Co. has been fined $22,500 for the deaths of migratory birds at a subsidiary's fluid pits in Montana, Wyoming and Nebraska, the U.S. Attorney's Office said. SM Energy pleaded guilty Wednesday to one misdemeanor count of violating the Migratory Bird Treaty Act in each state. U.S. Magistrate Carolyn Ostby also placed the company on probation for a year, fined it $7,500 on each count and ordered it to make a $7,500 payment to the National Fish and Wildlife Foundation. "These charges arose after a multi-year pattern of bird deaths at company sites, notification by the Fish and Wildlife Service, and failure of the defendant company's subsidiary to take reasonable measures to make the hazardous sites bird-safe," said Michael Cotter, U.S. attorney for Montana. The case dates back to 2005, when the U.S. Fish and Wildlife Service documented deaths of a dozen migratory birds at open fluid pits at a Nance Petroleum site in Wyoming. Billings-based Nance, which later became a subsidiary of SM Energy, paid a $3,025 fine. Oil and gas mining operations use temporary "reserve pits" to store fluid accumulated during drilling operations. The pits usually contain petroleum and other chemicals harmful to birds that land in the pits and ingest or become coated with the fluid, prosecutors said. Such pits can be covered with nets to prevent birds from coming into contact with the fluid. In 2007 and 2008, USFWS agents conducting follow-up investigation at Nance Petroleum sites in Montana, Wyoming and Nebraska found dead migratory birds and reserve pits that remained uncovered or insufficiently netted. "In all these cases, the birds died as a result of contact with liquids harmful to wildlife in the pits," the U.S. Attorney's Office said in a statement. In 2009, Nance began to address the bird mortality at its operation sites and hired a consulting company to develop appropriate measures to reduce the hazard to migratory birds. In November of that year, Nance merged with SM Energy.