I was painting a beach scene for a flightless shore bird exhibit for the renovation of the aviary at the Milwaukee County Zoo in 1993. The Zoo folks seemed pleased with what they were seeing, commenting on how realistic the sand dunes looked. “Like you could just run up them,” they said.

It was a fairly open exhibit with minimal barriers between the birds and the visitors. Near the end of the job the head bird guy, Ed, decided to let the shore birds — little long-legged brown-and-white-speckled guys called Cape Thick-knees — into the new exhibit to see if they could escape into the hallway.

I had to go back into their exhibit for some touch-ups, so I gently waded through the chirping flock. The birds were completely indifferent to my presence once they determined that I wasn’t going to feed them. While I was working, someone across the hallway knocked over a folding ladder with a loud “bang!” The startled flock of shorebirds turned away from the sound and ran smack into the mural, most of them falling over or wandering about, dazed.

The workman who had dropped the ladder saw this. He came up to the visitor rail and said, “Hey, I bet they were trying to run up that sand dune you painted!”

I looked, and sure enough, the spot they plowed into had the illusion of a gently sloping dune.

I was feeling pretty smug about my skills, but then other loud noises caused the little guys to run full-bore into the wall a few more times. When Ed stopped by, I said, “Guess what just happened! My sand dunes are so convincing, these bids are trying to run up them!”

He frowned and said, “I dunno, Paul, they run into walls all the time when something spooks ’em.”

“Oh,” I said, a bit crestfallen, “so you think it’s got less to do with my illusion and more to do with what?”

He grinned. “Your sand dunes look fine to me but maybe a better explanation . . . well, does the term ‘bird brained’ mean anything to you?”