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Decline of Red-Tailed Hawks Has U.S. Scientists Puzzled

Hillary Mayell
for National Geographic News
August 9, 2001
 
Raptor populations are declining in one of the most protected landscapes
in the country. Wildlife ecologists are asking why.

Although
red-tailed hawks are one of the most common birds of prey in North
America, their reproductive capacity in Jackson Valley, Wyoming, has
been on a 50-year slide.

Jackson Valley borders Yellowstone and
Grand Teton National Parks, both of which have extensive environmental
protection programs.



"There's no evidence to show that the activity of man is implicated in the reproductive decline here in the Jackson Valley," said Derek Craighead, director of Beringia South, a nonprofit ecology research and educational organization.

"So, the larger question becomes: If the most common raptor is suffering a population decline in this protected landscape, what's happening to other species, and to the ecosystem as a whole?"

In an effort to find answers, Craighead and wildlife biologists Roger Smith and Steven Cain have spent the last eleven summers traipsing through woods, mapping nesting sites, climbing trees, counting eggs, and banding chicks.

The problem is complex in part because even some of the most basic information about red-tailed hawks is little known.

"Despite the fact that they're one of the most common raptors in North America," said Craighead, "we don't know how long red-tailed hawks live in the wild, at what age they reproduce, whether they mate for life, or even what portion of the population pairs and nests.

"We don't even know where they go when they migrate south," he added. "We can make some guesses, but no one knows for sure."

Parasites, Pesticides and Productivity

The biologists' work is based on five decades worth of data—an unusually long span of time in wildlife ecology studies. Craighead's father and uncle began compiling data on the wildlife of Jackson Valley 50 years ago.

The present team has documented a decline in the productivity of the birds—measured by the number of chicks that live after they leave the nest—over the last 50 years.

The scientists found that 25 percent of the adult nesting pairs never laid eggs, 25 percent of all eggs that were laid never hatched, and 20 percent of the young that hatched didn't survive. Of those chicks that did survive, 37 percent died within three weeks of leaving the nest.

"What that means is that for the breeding season, each red-tailed hawk nesting pair produced only 0.9 young," said Smith.

A healthy number, the scientists said, would be in the range of 1.5 to 1.8 chicks per nesting pair. It also appears that the number of unpaired red-tailed hawks is larger than previously thought.

Among the suspected causes of the decline in productivity are the ubiquitous black flies that are the bane of fishing enthusiasts in North America. In some years, the flies may account for 30 to 40 percent of the mortality rates of the red-tailed hawks in Teton, Cain and Smith suggest.

"The flies are crawling all over the birds, taking their blood meal—basically eating them alive," said Craighead. "We would find fully feathered birds dead at the base of a tree, with necrotic skin around the throat."

The flies may also be transmitting a parasite, leucocytzoan, into the bloodstream of the birds, which can weaken or kill young birds. Weakness hurts their ability to protect themselves.

"The 37 percent that are dying within the first 10 days of flight are spending a lot of time on the ground, which makes them more vulnerable to predators," said Smith.

The parasite may also be affecting the older birds' ability to reproduce.

Tracking Migration

Another possibility is that something is affecting the birds at their wintering grounds.

"We wondered whether the areas were becoming more urbanized, or if herbicide and pesticide use was becoming more prevalent. Any number of factors could be causing a degradation or loss of habitat," said Craighead.

Trying to figure that out was even more difficult because no one really knew where the birds went for the winter. To find out, Craighead and Smith strapped mini-backpacks on several birds in the past two years. The backpacks contain a microchip that transmits data using satellite signals.

"We were stunned to see where they were going," said Smith. "We thought maybe southern California or Colorado, possibly even Texas or Nebraska.

"But instead," he said, "we found this totally undocumented path 2,000 miles south. One bird went all the way to Nicaragua and back." So the researchers visited Durango and Monterrey, Mexico, the winter grounds of two of the birds.

Red-tailed hawks like to nest and perch in high places, and they often feed on rodents in farmlands. But the availability of areas where the birds can prey and hunt is being reduced considerably, Craighead explained.

"Many of the areas that used to be family-owned farms—with fields broken up by hedgerows of native vegetation and windbreaks of tall trees—have now become industrial farms, with miles and miles of monolithic crops," he said.

Smith added: "We also saw workers walking up and down the fields spraying 2,4-D [a toxic herbicide] with no gloves, no face masks. It raised a lot of red flags as to whether herbicides and pesticides might be a factor."

As they continue searching for answers, the scientists say that probably no single factor explains the decline of red-tailed hawks.

"Right now we have this big ecological puzzle with many, but not all of the pieces on the table," said Craighead.

What the research does show, he said, is that the predator decline isn't just a "backyard" problem. "It's a problem of international scope, with birds traveling to Mexico and other Central American countries," he said.

Because of this, he suggested, management plans need to be developed on a much larger scale than government agencies have a mandate or even the funding to do.

"That's beginning to change," Craighead said. "Our job," he added, "is to find and assemble the information that gives us the insight and ability to manage our resources wisely."
 

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