In 2015, Tyson Foods severed ties with a supplier after Mercy For Animals published videos showing that employees at a Tyson supplier were stabbing, clubbing and stomping on chicken. A 2016 undercover investigation by the animal rights organization Compassion Over Killing showed workers at four separate Tyson processing plants throwing, punching and kicking chickens as well as sticking plastic rods through their beaks. They also wrung birds' necks, ran over them with forklifts and left injured birds in heaping piles to die. A 2017 investigation showed more abuse and cruelty towards chicken. After the 2017 investigation, Tyson responded by saying it would introduce a remote video auditing system to monitor treatment of chickens in its supply chain and hire off-site auditors. The company also started a pilot program for controlled atmosphere stunning, considered to be a more humane method of slaughter. Animal rights activists said the measure did not go far enough. In 2017, Matthew Prescott of the Humane Society of the United States criticized Tyson for failing to implement many of the animal welfare standards that other food suppliers were adopting.