(Editor's Note: This story first appeared in the February 2004 issue of Mammoth Monthly magazine. Subscribe here.)

When Adrienne Disbrow showed up at a meeting of a local government body in December, she was carrying happy news, indeed.

For those following the California Fish and Game's experiments with Swainson's hawk migration patterns, she announced to the Mono County Collaborative Planning Team, a small cadre of our banded feathered friends from the Eastern Sierra have arrived, safe and sound, in Argentina for the winter.

Not that they were exactly birds of a feather.

"They all took slightly different routes," said Disbrow, an environmental scientist with Fish and Game. "Two of the birds left the area in August, which was earlier than we would have expected.

"Two of the birds, both adults, flew north, to northern Nevada, before heading south. The other bird flew to Texas and stayed there several weeks before moving south."

In addition, she said, "They arrived in Argentina over a time of mid- to late November and early December. At my last check, they were still moving south in Argentina.

"And the birds did not leave here and move directly to Argentina. They flew to various locations and spent up to several weeks in other areas before continuing their move south."

The tracking of the local Swainson's hawks is important because this particular hawk has been listed as threatened under the California Endangered Species Act.

Using tiny satellite transmitters that are banded to the birds, Disbrow and her fellow scientists hope to discover what happens to local juveniles after their first migration.

This, along with nesting information that has been collected over the past six years, will help explain the population dynamics, nesting success, and juvenile survivorship of the species in the area, she said. In turn, she added, this information will help determine if specific management issues exist in the protection of the species.

The Swainson's hawk (Buteo swainsoni) is distinguished from other hawks by long, narrow, pointed wings. It is a gregarious bird that migrates in large flocks. In this case, the local hawks joined others in the Great Basin before heading south.

It is the most mobile of our local hawk populations, Disbrow said, and explained that scientists still are studying why some hawks migrate and others don't.

"One theory is that migration evolved as a result of climatic changes and adaptations associated with those changes over thousands of years," she said.

"Ecologically, it appears that birds migrate to take advantage of different favorable breeding and foraging conditions that exist at different times in northern and southern latitudes. In addition, each hawk species has different, specific needs and hunting strategies, and some species may be able to meet these requirements without migrating, while others cannot.

"In our area and in others, there also appears to be some division of resources occurring. Some hawks breed and overwinter locally, while others, like the Swainson's hawk, breed locally and overwinter elsewhere.

"Still others nest much farther north and actually winter here," she said. "All the hawk species in the area could not thrive if they all occupied the area year round."