Police have stepped up patrols after the incident on the Hawksworth Wood Trail near Horsforth in Leeds on Friday afternoon.
The witness, posting in the community Facebook group for residents of the Hawksworth estate, claimed she had seen a 'ritual' taking place in the local beauty spot while walking in the woods with a friend.
"We saw a naked couple on a white plastic sheet having sexual activity while chanting and praying with a dead bird at the side of them.
"The man then wiped blood from the bird on the woman's head while she lay on the ground.
"Other people were there who seemed to be watching.
"They didn't seem bothered that they'd been seen. It was a really odd situation to stumble upon."
West Yorkshire Police said: "Police received a report of public indecency at Hawksworth Wood Trail on Cragside Walk in Leeds yesterday afternoon.
"Police have stepped up patrols in the area."