The mission of the roughly two dozen animals: to mow the grass. And lots of it.
O'Hare is one of the largest airports in the world and takes its environmental initiatives to serious and sometimes quirky heights. It has acres of green roofs, including one atop an air traffic control facility, to reduce storm water runoff and lower the urban heat island effect of the airport's massive concrete expanse. The airport has even turned over a wooded patch of land to 1 million bees living in 28 beehives that produce honey sold in the terminals and help replenish declining bee populations.
"Welcome to Project Herd!" said Rosemarie Andolino, head of the Chicago Department of Aviation, announcing the new effort to a group of journalists who got a look at the project Tuesday.
Behind her, the goats and their furry friends were munching their way through a steep embankment overgrown with tall grass and cattails on the far northeastern corner of the 8,000-acre airport. Two bushy llamas bounded up to the top, chased by one of the herders charged with looking after the animals.
Under the mid-afternoon sun, the animals happily grazed or dozed, seemingly oblivious to the roar of jumbo jets taking off and the jostling of the gaggle of news photographers and television reporters, who outnumbered the animals.
One of the sheep had just given birth to a lamb. The little guy, named O'Hare, was nuzzling its mother when reporters arrived.
"He's doing great. He was suckling on mom," said Pinky Janota, who donated some of the animals from her rescue shelter in Beecher, Ill., south of Chicago, and helps manage them on site. "Planes flying overhead; he didn't flinch. Mom didn't move. Everybody's content."
Other airports have similar programs, including at San Francisco International, which uses a company called Goats R Us to clear brush each spring in an effort to protect nearby homes from potential fires. The other airports are in Atlanta and Seattle.
At O'Hare, the main goal is to rid the airport grounds of habitat for birds and other wildlife that can present a serious hazard to departing and landing aircraft. Many of those areas are beyond the reach of traditional mowing equipment, which can't handle the steep embankments or the rocky and loose soil.
Rabbits that hide in the grasses also draw birds of prey such as red-tailed hawks. Deer and other animals wander into the area along the region's many railroad tracks, which act as pathways for wildlife.
To scare away coyotes, there are the no-nonsense llamas and burros.
"The wild burros chase them and stomp them to death," Janota said.
So where does an airport find a herd of goats?
The Department of Aviation's want ad got a lively response from interested herders and set off a bidding war. The contract, which amounts to just under $20,000 for two years, went to Central Commissary Holdings LLC, which was raising a small goat herd to produce cheese for its Chicago restaurant, Butcher & the Burger.
It supplemented the herd with animals from Janota's organization, Settler's Pond Animal Shelter.
The project will lower the landscape maintenance costs for things such as fuel and labor, and offer an alternative to using toxic herbicides that can spill off into waterways.
But airline passengers needn't fear a high-speed collision with a foraging critter. The herd will be kept far from active areas of the airfield or behind fences.