KANE'OHE BAY — Twenty-seven tons of tank-treaded armor crashed through Nu'upia Ponds yesterday, carrying out the delicate task of environmental restoration.

That's the unusual symbiotic relationship that's gone on now for a quarter-century at the Marine Corps base.

Once a year, Marines get to drive their 27-ton amphibious assault vehicles to their heart's content, and a range of endangered bird species benefit from the crushing force.

The Marine vehicles flatten invasive pickleweed, and create mud mounds that are perfect for nesting by birds like the endangered Hawaiian stilt.

"What we're doing is improving the habitat for several of our endangered species and also some protected migratory waterfowl that make Nu'upia Ponds their home," said Diane Drigot, senior natural resources manager for the Marine Corps base.

The Marines just know that for three days, they can drive like madmen (there is a 25 mph base speed limit) through a lot of water and mud.

"It was fun. Dirty," said Pfc Juan Hernandez, 26, a driver from San Antonio, Texas. "Drive a big vehicle. Run over things — in a good way."

If left unchecked, the pickleweed grass that was an import from South America "literally crowds the birds out of house and home," Drigot said. "And if we don't go in there every spring like a farmer plowing his field, the vegetation will just overtake the habitat."

The "mud ops," as the Marines call it, is conducted every January or February, ahead of nesting in May and June.

Drigot said the Hawaiian stilt count has gone from 60 to 160 birds at the roughly 500-acre wetlands. Other species also have benefitted.

"We've seen two families of Hawaiian coot, which is another endangered species, and we have the koloa duck which have come to make these ponds their home," she said.

Jeff Mikulina, executive director of the Sierra Club Hawai'i Chapter, said the amphibious vehicles help keep pickleweed at bay.

"As long as we have military training on the island, it's going to entail significant environmental tradeoff," Mikulina said. But he added that the Marine Corps base, in particular, "has done a fairly good job of finding ways to give back."

The pickleweed/training program grew out of the Marines' observation that birds like stilts were nesting atop the furrows they created after driving through the ponds.

A separate sea access was created, and they don't drive through the ponds anymore.

Gunnery Sgt. John Duncan, 39, from Woodland, Calif., said the Marines get plenty of sea training with the combat assault company's 15 amphibious vehicles, but land training is limited to once a year at Nu'upia Ponds, a trip to Pohakuloa Training Area on the Big Island, and excursions to Schofield Barracks.

The ride on land is much different than at sea.

"We don't have much room to drive around, so being here (at the ponds), it does help," Duncan said. "There are hills and rocks and stuff that we have to maneuver around."

Reach William Cole at wcole@honoluluadvertiser.com.