By Shelby Sebens

PORTLAND, Ore. (Reuters) - As planes took off and landed behind a fence one unusually warm April day at Portland International airport, a herd of goats lay basking in the sun, taking a nap break from their 24-hour job of chowing down on invasive plants.

Soon afterwards though, the 40 Spanish and Kiko breed goats were back at work, using their hoofs to hold down blackberry brambles and their nimble lips and sharp back teeth to cut away unwanted foliage.

The airport took on the goats, watched over by a shepherdess and a protective brown llama named Monty, to clear a large patch of overgrown land just outside the airfield that is inaccessible to mowers.

The goats, owned by Portland company Goat Power LLC, are accustomed to traveling all over Oregon to clear weeds from vineyards, schools and private yards.

"This is our first airport," shepherdess Briana Murphy told Reuters as some of the goats, with names such as Moon and Chili, rubbed her leg and begged for attention.

If they are stuck at home eating hay, "they get really annoyed," she added. "They actually really like working."

The goats were scared of the planes at first, she said, but within days became used to the noise, hardly blinking when an F-15 fighter jet thundered overhead. They will spend three weeks at the airport and clear 5 acres (2 hectares) of land.

While they work, Monty the llama keeps a watchful eye out for predators, ready to call out an alarm and able to kill a coyote with one kick.

Environmental workers at the airport saw the goats as an opportunity to avoid using harsh chemicals.

"Portland loves its goats. So it wasn't really a tough sell," Port of Portland environmental technician Matt Paroulek said.

And they are not the first animals to work at PDX, the airport's call letters. A border collie named Fish also chases away geese, and there are 29 beehives kept on airport property for research into breeding a Pacific Northwest queen bee. 

The aim is to give native plants, which the goats will not eat, a chance to thrive. It will also prevent coyotes, which climb barbed wire fences to get on the runway, from making dens in blackberry bushes.

The airport is still getting used to its temporary workers. As the goats munched vines, one surprised airport employee driving by backed up his truck and yelled: "There are goats!"

(Editing by Daniel Wallis and Eric Walsh)