It was hard to tell who was having more fun in the mud — the Marines in their 26-ton amphibious assault vehicles or the endangered Hawaiian stilts reveling in the churned-up food.

For 30 years, the Marine Corps at Kaneohe Bay has had a symbiotic relationship with the stilts and other endangered water birds that make the adjacent Nuupia Ponds Wildlife Management Area their home.

Once a year for a few days, the Marines get to tear around the marshy shoreline in their tracked vehicles — tearing out invasive pickleweed, dredging up crustaceans and minnows for the birds and creating mud mounds in the process that are perfect for nesting Hawaiian stilts.

"It’s a really good example of being able to partner between achieving combat readiness and conservation," said Diane Drigot, senior natural resources manager for the Marine Corps base.

Pickleweed takes over the shoreline each year, and the Combat Assault Company AAVs mash it down to create nesting mounds surrounded by water channels for the birds.

There are only between 1,000 and 1,500 black and white Hawaiian stilts in the world — all in Hawaii — with about 150 of them, or about 10 percent, at Nuupia Ponds, Drigot said.

The number has grown with the "mud ops" exercises. When Drigot came to the Marine Corps base in 1982, there were about 60 stilts there, she said.

On Wednesday, the Marines were on day two of the three-day exercise to navigate mud that sometimes is 10 feet deep, getting stuck occasionally and practicing towing as well.

It’s different from the shore-to-ocean training they do about once a month with the 16 AAVs at the base.

"It’s good for the recovery aspects and for our young drivers," said Capt. Dan Petronzio, the commander of the Combat Assault Company. "Living in Hawaii, we don’t have a lot of land-driving opportunities and terrain that we would on the mainland, so when we can get out on the mud flats and get the drivers a unique training experience, it’s always very valuable."

The unevenness of the terrain Wednesday caused the big vehicles to rock back and forth and side to side as they navigated the marshy shoreline, kicking up rooster tails of thick gray mud. One Marine likened it to a roller coaster ride.

"They love being out here tearing stuff up. I mean, it makes you feel like a 12-year-old. You get to stomp around in the mud with a big vehicle. I can’t believe I get paid for this, so it’s pretty amazing," Petronzio said.

About two dozen stilts were checking out the water troughs created by the AAVs on their passes through the ponds area, as were some Hawaiian coots — also endangered, Drigot said.

Two other endangered water birds, the Hawaiian moorhen and koloa duck, also make the ponds home, she said.

The "mud ops" exercise grew out of regular AAV training use of the Nuupia Ponds waterways during the Vietnam War, Drigot said.

"This was their regular highway," she said.

Even though the Marines really messed up the terrain, water birds loved it, so much so that the Marines called up state and U.S. Fish and Wildlife Service officials and said, "Can you please get these birds out of our way — they are in our way," Drigot said.

An agreement was reached to use the ponds for some training, which improved water bird nesting, she said.

"When you think out of the box, you come up with what appears on the surface to be ideas in conflict, and you make them work so everybody benefits," Drigot said.

Most of the important wetlands on Oahu are on the Windward side, and nearby Kawainui Marsh also is full of weeds, Drigot said.

"They are trying to improve it and help the stilt and other birds, but they have a long road ahead," she said.

Drigot said the AAVs can’t really be used for the same habitat benefit elsewhere "because as the way the legal (office) explains it to me, we’re forbidden from using military vehicles on nonmilitary land because we’re competing with the private sector for jobs." The exception would be for natural disasters.

The Marines, it seems, love the mud at Nuupia Ponds as much as the stilts.

Lance Cpl. Mitchell Van Ryn, 21, who was driving Wednesday, said it was "an awesome experience."

"It’s fun. At the end we usually all just jump in the mud and have a good time, and at the same time, we’re helping the environment," Van Ryn said.