BOSTON — The sight of Pat Mehlhop slogging through a willow thicket along the Rio Grande is enough to make many Southwest ranchers nervous.
That's because Ms. Mehlhop is looking for the nests of the Southwestern willow flycatcher - a species that some biologists call the most endangered songbird in North America. And the more she pokes around, the more she is concerned by what she sees.
What she has found - cowbird eggs in many flycatcher nests - has triggered a ban on cows on federal lands in the river basin and set off firestorm of protest among Southwest ranchers who already feel beleaguered on environmental issues.
In a conflict that echoes other endangered-species battles in the American West, the three-month ban may be a first step in a dramatic cutback on public-lands grazing along rivers and reservoirs where the flycatcher lives in New Mexico and Arizona. Indeed, the Forest Service says next year it will begin removing or restricting livestock from portions of the 21 million acres it manages that are critical to the bird's survival.
The ban on cows, environmentalists hope, will keep out cowbirds, which eat the seeds and insects churned up by tromping hooves. Cowbirds' danger to flycatchers, however, comes from their propensity to lay their eggs in other birds' nests and then abandon them. Their progeny hatch quickly, generally before the young of the nest-builder, and they are large and aggressive, taking the lion's share of food brought by the unsuspecting songbird parents. Often, the songbird young die while the cowbirds reach maturity.
For the southwest willow flycatcher, which nationwide number less than 1,000, cowbird parasitism is a potentially huge problem added on top of the degradation of much of its nesting habitat along rivers. And for a tiny colony of birds, such as the one at Elephant Butte, it could quickly lead to extinction.
"There's just not enough of them to survive both parasitism and the loss of riparian habitat," says Mehlhop.
Based on Mehlhop's work, the Bureau of Reclamation, which manages Elephant Butte and 6,000 acres of surrounding lands, began trapping cowbirds in 1996. Over two months, biologists caught an astounding 1,200 birds.
But this spring's even bolder move - kicking the cows off three federal grazing allotments - was in response to more than just gathered data. The Santa Fe-based Forest Guardians threatened the bureau with a lawsuit, claiming that the agency would be illegally harming the flycatcher, which is protected under the Endangered Species Act, if it allowed grazing in the area.
Ranchers say they were caught by surprise. "The decision was just a superfast reaction that was based on weak scientific data," says Harlen Smith, a former area manager for the Bureau of Land Management assisting one of the affected ranching families.
Science is central to the ranchers' appeal of the decision. They claim there is no proof cows have brought in the cowbirds that have parasitized the flycatcher nests. They also point to a study done along Arizona's Gila River where cows and flycatchers appear to both be thriving.
In fact, only a few flycatchers currently inhabit the 21 million acres of national forest in the region, but there are hundreds of miles of unoccupied, potential habitat along streams. And cattle graze virtually every acre.
Bud Eppers, president of the New Mexico Public Lands Council, which represents 3,500 public lands ranchers in New Mexico, says he worries the Forest Service restrictions could drive some ranchers out of business. They will have to purchase additional pasture and water for their animals. Ranchers say they may go to court to fight any further restrictions on grazing.
John Horning of Forest Guardians, says removing cows from would hurt some ranchers, but it is "crucial to the species' survival."