You take your family to the beach and to the nearby bird sanctuary at Montrose Avenue. Instead of enjoying nature and seeing beautiful birds, though, you end up seeing something shocking.

In response to viewer complaints, 2 Investigator Pam Zekman checked out what’s going on.

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The 25-acre designated nature preserve right along the lakefront has paths people use to go back and forth from Montrose Beach. The area also attracts beautiful birds and people who like to watch them and enjoy nature.

But it’s also become a haven for something else.

“If you see men walking around out here and they’re not doing anything, they’re not carrying binoculars or fishing rods … then they’re almost certainly cruising, looking for sex,” says bird watcher Robert Hughes.

Hughes is among the many birders who complained to CBS 2 people having public sex in Montrose Park.

“It’s a nature sanctuary, it’s not an outdoor brothel, and we’d like to keep it that way and get the cruiser problem under control,” he said.

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Fellow birder and Chicago Police Det. Louis Munoz agrees.

“You come out here after 10:30 you’re gonna see all these guys here coming in and out,” he said.

He showed Zekman traces of the activity, including condoms and condom wrappers. CBS 2 watched with hidden cameras for several days as men circled the path. They weren’t looking for birds, they seemed to be looking for a hookup.

Munoz says he’s caught many people in the act.

“I told these guys, ‘Get outta here, I’m calling the police,'” he said. “I take out my cell phone, and these guys took off like rabbits – both of them. The one guy was pulling up his pants.”

“I bring my kids here, I bring my daughter here, I bring my two nephews here,” Munoz said, explaining his anger. “They should not have to see this junk at a nature preserve.”

Police officials say they have started cracking down on offenders, charging them with public indecency. As a result of an earlier 2 Investigators report about a similar problem behind the Museum of Science and Industry, the Chicago Park District is installing new lights and a camera to help deter public sex acts.

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Their advice to anyone who sees it: Call 911 and report it.