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

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"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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