Chicago - They're the rock stars of the bird world these days, the Rolling Stones of the feathered set. But the penguins at the Shedd Aquarium are showing it's not all film premieres -- think "Happy Feet" and "March of the Penguins" -- and sushi. It's rocks. Real rocks a bird can build a nest out of.

A Rockhopper Penguin swims beneath a colony of penguins as the 'battle of rocks' begins at the Shedd Aquarium in Chicago, Wednesday, April 11, 2007. The week long 'battle' begins when marine mammal trainers deposit dozens of small nesting rocks in the exhibit for the Gentoo and Rockhopper penguins to fight over and use for their nests.[AP]

On Wednesday, the keepers at the Shedd started rolling out the rocks for the Gentoo and Rockhopper penguins who call the aquarium home. For these types of penguin, nothing says romance like a pile of rocks.

"What's going on here is a very exciting day for the penguins," said Gretchen Freimuth, senior marine trainer at the Shedd. "This actually cues the birds to start pairing up and building those nests."

Freimuth acknowledges that some birds didn't wait for keepers to throw down the rocks to let them know it's time to do something besides swimming and eating. But, she said, "Once the rocks are put out we will see a lot more of that activity."

The process isn't something the birds cooked up at the aquarium; it happens every year back home in the Antarctic, according to the Shedd.

Freimuth said the love nests range from simple affairs of about 15 rocks to bigger nests made of big piles of rocks. The penguins will go after a rock they like even if it's already part of another penguin's nest.

"They will steal from one another," she said.

The staff will put out rocks for about a week, according to Shedd spokeswoman Melissa Kruth, pointing out that both males and females participate in the nest building.

After that, the mating begins in earnest and eggs start to appear about 23 days or so later.