The 35-story building in Grand Circus Park is being redeveloped from unoccupied office space into 127 residential units. The work is expected to be finished in September.

The current ad banner will be removed by Oct. 1, Beal said, and the whale mural will remain visible for "an extended period of time."

City architecture aficionados consider the mural a unique part of the landscape.

"There's no doubt the thing is a Detroit icon," said Dan Austin, founder of the nonprofit site HistoricDetroit.org and author of Lost Detroit, a book that delves into the history of the city's skyscrapers, including the Broderick.

"In many ways, the whales have become more of a landmark than the Broderick Tower itself," he said. "After being dark and silent for decades, the building's name was largely forgotten.

"I remember telling people that they were going to start renovating the Broderick, and they'd have no idea what I was talking about. I'd say, 'You know, the building with the whales on it,' and they'd say, 'Oh! That one!' "

The building, constructed in 1928, is attractive to advertisers because it overlooks Comerica Park, home of the Detroit Tigers and thousands of fans who see the building for 81 games a season.

"Later on, new and narrower -- so they don't cover the windows -- commercial advertising banners may go up from time to time," Beal said.

The mural, on the tower's eastern wall, falls under the Visual Artists Rights Act of 1990, the federal law that protects public art, so it cannot be purposely destroyed or altered, Wyland said.

But it can be covered.

The Detroit Board of Zoning Appeals approved the variance in 2004 that allows the ads on the Broderick Tower.

In 2006, the first massive vinyl banner, advertising the Jeep Compass, was hung on the building, completely obscuring the mural.

Wyland's original plan was to paint whales on Joe Louis Arena, but that met too many objections, he said. A deal was eventually worked out for him to paint the vacant Broderick.