O'HARE — The job of landscaping the more than 11-square-mile expanse of O'Hare Airport is too much for humans to handle on their own.
For the fourth summer in five years, the nation's second-busiest airport has turned to a herd of goats and sheep to clear the shaggy brush surrounding its miles-long perimeter, a tool city officials say is both more efficient and environmentally-friendly than heavy machinery.
This year's herd includes 100 animals, roughly doubling the size of its goat army over previous years, officials said.
The city's aviation department still uses human-operated mowers and tractors for the flatter patches of grass buffering the airport's runways and tarmacs, according to Aaron Frame, the department's deputy commissioner of environment.
But the jagged terrain and thorny overgrowth just outside the airfield call for a different kind of landscaping crew.
In less than a month, the herd has torn through almost 16 acres of willows and weeds at the northern edge of the airport grounds, leaving a barren plain of buzzed grass in their wake.
The goats are unique for their ability to "eat just about anything," according to Ben Robel, a shepherd with Wisconsin-based Vegetation Solutions LLC, the company contracted by the city.
"They evolved in rocky environments without a whole lot of vegetation, so their stomachs are built to digest things that cows and other animals would find toxic," Robel said.
They can also be more thorough than mowers, gobbling up seeds and roots instead of just snipping their stems, according to Greg Martinelli, a wildlife biologist with the U.S. Department of Agriculture.
Martinelli is one of three biologists employed by the airport to scope out nearby wildlife and keep it from interfering with arriving or departing planes, he said.
"We can sometimes get birds or smaller mammals going across the runway, so we're trying to eliminate cover for them," Martinelli said. "And those are the kinds of habitats that are the hardest to get to with machinery."