CHICAGO — Undercover animal rights activists exposed shocking cruelty to chickens at McDonald's egg supplier, prompting the fast food giant to cut ties to the US firm after the video was released Friday.

"Unfortunately, much of the abuse we documented is not only standard, it's legal," said Nathan Runkle, director of Mercy for Animals.

There are no federal laws governing the treatment of poultry on US farms and most states have sweeping exemptions for farmed animals which allow for abuses to run rampant without prosecution, Runkle said.

The situation is not much better for cows and pigs -- which are at least supposed to be stunned before slaughter -- because of a lack of oversight, he added.

"We've done over a dozen investigations at factory farms from coast to coast," Runkle told AFP.

"What we have found is that every time we've sent an investigator into one of these facilities they've come out with shocking evidence of abuse and neglect."

The latest video shows chicks having their beaks being burned off by a machine and then tossed into cages and barely identifiable corpses of chickens that were left to rot in cages.

It also showed unwanted chick left to die in plastic bags, chickens mangled by the bars of overcrowded cages, and a plant worker walking down the hall swinging a live chicken on a rope in a wide circle as it flaps its wings in distress.

"The behavior on tape is disturbing and completely unacceptable," McDonald's said in a statement.

"McDonald's wants to assure our customers that we demand humane treatment of animals by our suppliers. We take this responsibility -- along with our customers' trust -- very seriously."

The fast-food giant said it has directed its supplier, Cargill, to stop sourcing McDonald eggs from Sparboe, which insists that the video depicts acts that are "totally unacceptable and completely at odds with our values."

The family-run egg company said it launched an internal investigation after learning of the video and has fired four workers who engaged in mistreatment of the chickens.

In a message posted on a dedicated website, owner Beth Sparboe Schnell said an independent auditor from Iowa State University confirmed the company is in "full compliance with our animal welfare policies."

She noted that Sparboe Farms was the first US egg producer to have its "science-based animal care production guideline" certified by the US Department of Agriculture.

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