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.