O’Hare International Airport is the busiest airport in the country and operates on 7,000 acres. The land is more than runways and terminals, there’s lots of overgrown vegetation that need to be trimmed back in the spring and summer. Instead of diesel- or gas-powered landscaping equipment, the airport brings in goats, sheep, donkeys, and, at one point, llamas to graze on nearly 200 acres.

The shepherd and a herd of goats, sheep, and a donkey named Jackson will return at the end of May. The program is back for its sixth season and is the longest-running grazing herd at an airport.

In 2013, the Chicago Department of Aviation (CDA) gave the contract to a Chicago-based farm of Illinois rescue animals. That herd was especially social and loved attention from humans. Before they arrived, the team was worried that loud jet engines or trucks would spook the animals.

“Surprisingly they weren’t phased by the airport noise at all,” Aaron Frame, CDA’s Deputy Commissioner of Environment, told Curbed Chicago.

The following year, a working herd of 50 sheep and 50 goats were brought in and those animals were more skittish around humans. It was clear that group was just there for the abundant grass, weeds, and leaves.

Last year, the animals came from a farm in Lemont, Illinois and some of them will be back in May including Jackson the donkey. In five to six months, the herd won’t get through all 200 acres but the grazing that gets done is important.

Lawnmowers and trimmers take care of the landscaping most passengers see on their way to O’Hare. The grazing herd sticks to the rocky and hilly areas around the Willow-Higgins Creek that are more difficult to maintain in a traditional way. The vegetation that’s eaten removes habitats for birds and squirrels that wouldn’t be safe so close to an airport. Also, the herd is an alternative to toxic herbicides and doesn’t emit carbon dioxide like gas-powered equipment. City workers don’t have to collect and transport debris to landfills either.

There’s a lot O’Hare is doing to minimize its impact on the environment besides the grazing herd. Six buildings are LEED certified and there are 10 acres of green roofs. Landscaping around the airport is native and drought tolerant. An apiary was set up on a previously vacant grassy area and now the bees produce hundreds of pounds of honey every year (airport restaurants use the honey and there are beauty products sold online).

The CDA has prioritized going green and Chicago was the first in the U.S. to develop sustainability guidelines for the design of airports. Although it is Oslo, Norway that claims the world’s greenest terminal.