Roosting in a tree at the end of Richard Edwards' garden were three peacocks and three peahens but following a 'barbaric' attack, one was killed.
The incident took place on Thursday (January 23) in Slindon Bottom Road and has left Richard wondering what drove the attacker to do it.
Richard, who works as a veterinary surgeon at Alpha Pets Vets in Bognor Regis, said: "It's not a good situation. He wasn't dead. I had to euthanase him on Wednesday morning after my wife noticed he wasn't coming down from the tree and was being very slow.
"It didn't go back up [the tree] that night and we found it with the sheep which was unusual."
The five-year-old bird's feathers were fresh and ready for it to begin mating again later in the year, Richard said.
He said: "It was sitting in a tree and it was fired at from below. The ball went up through the beak, up through the tongue and into the top part of its face and landed behind its eye. It was undoubtedly in considerable pain.
"I thought it was a bite as they fight from time to time. I thought it had been unlucky.
"The following morning its tongue was hanging down through the hole the bullet left. It's utterly senseless. It has to be a catapult. It's barbaric."
A spokesman for Sussex Police said: "officers are investigating a report of a peacock being shot on Slindon Bottom Road, Arundel on Thursday, January 23.
"The peacock is believed to have been injured with a slingshot two or three days before.
"Anyone with information regarding this incident is asked to report online or call 101, quoting serial 1036 of 23/01.