What is hidden in 3D image?

Each evening bird-lover Tom stood in his backyard, hooting like an owl. One night, an owl called back to him. For a year, the man and his feathered friend hooted back and forth. He even kept a log of the "conversation."
Just as he thought he was on the verge of a breakthrough in interspecies communication, his wife had a chat with her next-door neighbor.
"My husband spends his nights... calling out to owls," she said.
"That's odd," the neighbor replied. "So does my husband."