Feds take on a sneaky species

  • A young cowbird (right) out-competes a songbird

    A. Morris, Vireo
 

Two years ago, Pat Mehlhop waded through a willow thicket on the shore of Elephant Butte Reservoir in southern New Mexico, carrying a 20-foot-long pole with a mirror attached to one end.

The ecologist with the New Mexico Natural Heritage Program was in search of what has become a rarity along the state's waterways: the nest of the southwestern willow flycatcher.

More than the cup-shaped nests themselves, Mehlhop was interested in what they contained. That's where the mirror came in. By extending it over a nest, she could count the eggs inside.

In the spring and summer of 1995, Mehlhop and her field team found six flycatcher nests on the northern edge of the reservoir. All held small, white-and-brown flycatcher eggs, but four of them contained larger eggs, too, that didn't belong.

"There's no mistaking a cowbird egg," says Mehlhop.

How did they get there? Female cowbirds lay their eggs - up to 40 or 50 a year - 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 young songbirds die while the cowbirds reach maturity.

It is a strategy that has proved wildly successful for the cowbird, which evolved with the nomadic bison on the Great Plains. When your food source - seeds and insects churned up by bisons' tromping hooves - moves around so much, there is no time for parenting. And though the bison barely survived the great Western settlement of the last 150 years, cowbirds have thrived on it, spreading wherever humans and their plowed fields and livestock have gone.

For the southwestern willow flycatcher, cowbird parasitism is potentially a huge problem, according to Mehlhop and other biologists. For a tiny colony of birds, such as the one at Elephant Butte, it could quickly lead to extinction.

"There's just not enough (flycatchers) to survive both parasitism and the loss of riparian habitat," says Mehlhop.

This spring, the Bureau of Reclamation, which manages Elephant Butte and 6,000 acres of surrounding land, took a bold step to protect the flycatcher from cowbirds, based in part on Mehlhop's work. It temporarily kicked the cows off three federal grazing allotments in hopes of reducing cowbird parasitism during the flycatcher's three-month breeding season.

The move was in response to a threatened lawsuit from the Santa Fe-based Forest Guardians. The group claimed 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. "They got the notice on May 9 and they had to get their cattle off by May 15. That's just five days to round up their herds and rent out some other pasture," says Harlen Smith, a former area manager for the Bureau of Land Management, who helped one of the ranchers move. "The decision was just a super-fast reaction that was based on weak scientific data."

Science is central to the ranchers' appeal of the decision, which is now before the Interior Board of Land Appeals. They claim there is no proof cows have brought in the cowbirds that have parasitized the flycatcher nests in the past.

They cite a scientific study done by two biologists for mining giant Phelps Dodge, which owns riparian habitat along the Gila River, the flycatchers' last stronghold. The study claims there is no connection between the presence of cattle and parasitism by cowbirds.

"In the three years of intensive survey conducted by us at the U Bar Ranch, the number of nesting southwestern willow flycatcher pairs has increased each year (63 pairs in 1994; 107 pairs in 1995; and 138 pairs in 1996) while livestock numbers within the survey area have remained unchanged or have increased," writes Dennis Parker, an employee of Applied Ecosystems Management Inc. of Flagstaff, Ariz.

David Leal, a U.S. Fish and Wildlife Service biologist based in Albuquerque, says the work of Parker is not peer-reviewed and does not reflect what other scientists have found.

Leal and other U.S. Fish and Wildlife biologists who recommended pulling the cows off remain firm. They claim there is a well-documented association between cattle and cowbirds.

"The cattle just stay in the riparian areas all year long," says Leal. But with the flycatcher in such trouble, he says, "it's hard to justify leaving the cattle in there during the breeding season."

Even before this year's decision to remove cows, federal managers at Elephant Butte had already begun taking steps to control cowbirds. Last year, at the urging of the U.S. Fish and Wildlife Service, the Bureau of Reclamation began trapping the birds. Using just five traps, Leal says, biologists caught 1,200 birds in two months.

Though a dozen pairs of flycatchers showed up that year, just one attempted to nest, and it failed to produce young. Leal says he suspects a drought was the culprit: The Rio Grande was bone-dry last summer. The flycatchers, he says, rely heavily on water-dependent insects for food; they also seem to prefer to nest in flooded areas, perhaps to keep ground-dwelling predators at bay.

This summer, water was more plentiful, but still only one nest produced young on the federal allotment. Ranchers did not give BuRec biologists permission to monitor their adjoining private lands, though some flycatchers had been found there in previous years.

Though this year's reproduction has been disappointing, Leal says the results of the cowbird trapping are cause for some optimism. After biologists laid out 15 traps over a four-month period, they caught just 400 birds, which was far less than last year's 1,200. The lower number could be due to a lot of factors, says Leal, "but keeping cows out of the allotment may have helped."

Barring a contrary decision from the Board of Land Appeals, Leal says his agency will likely recommend that cows stay out again next year during the flycatchers' breeding season.

Forest Guardians, meanwhile, hopes to take the concept of removing cows near flycatcher nests to other public lands. It recently issued a 60-day notice of intent to sue the U.S. Forest Service for failing to provide a buffer between cattle and birds. The lawsuit covers all Forest Service allotments in the Southwest within five miles of active flycatcher nests.

John Horning of Forest Guardians says the lawsuits are necessary because "for decades everyone has been asleep at the wheel when it comes to riparian habitat, and now every single pair of flycatchers is crucial to the species' survival.

"We get a bad rap because we issue the wake-up call," he says, "but ranchers and federal agencies bear much of the responsibility for the bird's decline."

Paul Larmer is HCN's senior editor.