When Marines tore through Hawaiian mudflats in heavily armed half-boat, half-tractor vehicles this month, they were doing much more than preparing for battle. The 26-ton battle machines that they were learning to use, called amphibious assault vehicles (AAVs), helped to create nesting habitat for an endangered wading bird, the Hawaiian Black-necked Stilt. At Oahu’s Nu’upia Ponds mud flats, non-invasive pickleweed grows like, well, a weed in the salty water, overtaking any possible nesting sites for the stilt. In the 1970s, the U.S. Fish and Wildlife joined forces with the Kaneohe Marine Corps Air Station to break up the thick mat of vegetation with AAVs in annual training exercises at the Wildlife Management Area. The machines act just like garden-variety rototillers (except, you know, they’re outfitted with machine guns and grenade launchers), opening up areas for the thin, pink-legged birds to lay eggs. Left to its own devices, the invasive pickleweed would cover the...