Tweets,
thought this was interesting. From yesterday's Sacramento Bee.

>Valley hawks' path startles researchers

By John D. Cox
Bee Staff Writer
(Published Dec. 29, 1997)
The first satellite tracking of the migration of Central Valley
Swainson's hawks has astonished California biologists. Unlike the main
North American population, the birds don't spend the winter in the
Pampas of Argentina -- or anywhere near it.

"Everybody who has gotten this news and knows anything about Swainson's
hawks is completely surprised," said Michael Bradbury, a wildlife
biologist for the state Department of Water Resources.

While the main population from the western plains traveled 5,000 miles
to the Argentine agricultural valleys near the tip of South America,
five of the six Central Valley Swainson's hawks equipped last summer
with radio transmitters never left North America.

Instead, the five hawks have settled into southern Mexico for the winter
while a sixth bird crossed over Central America and settled in southern
Colombia.

The evidence suggests that the California birds are more profoundly
isolated geographically, and therefore genetically, than researchers
would have guessed -- cut off from the rest of the species in their
wintering range as well as their nesting range.

"When we started this project, we would not have been surprised if they
had ended up down in Argentina with the main body of Swainson's hawks,"
Bradbury said. "We were at least confident that they were going to South
America, even if they weren't wintering with the main group.

"We're absolutely amazed that these guys are only in Mexico."

Representing a population of about 1,500 Swainson's hawks, the data from
the six birds is another argument for the unique character of the
Central Valley birds, an issue that could be central to the evaluation
of any future protections for the birds.

The population that nests in the Central Valley from March through
August is classified as threatened under the state's Endangered Species
Act, although it has no protection under the more strict federal
statute.

Swainson's hawks once ranged over much of the state, but a century of
development has seen their numbers decline by 90 percent and their range
shrink primarily to the agricultural and Delta lands of Yolo, Sacramento
and San Joaquin counties.

"We wanted to know if, genetically, they were wired the same way as the
main body of Swainson's hawks," Bradbury said. "And if that were the
case, we would expect them to migrate the same way and end up in nearly
identical places.

"We already had some evidence that suggests that these birds are
genetically different, and this is just another very strong piece of
evidence."

The satellite data has a more immediate impact on scientists studying
the birds.

The La Pampa migration of the main population of about 500,000
Swainson's hawks was discovered just three years ago by U.S. Forest
Service ornithologist Brian Woodbridge, using the same satellite
telemetry technology.

When Woodbridge traveled to Argentina, he found thousands of Swainson's
hawks dying of pesticide poisoning, a finding that prompted a continuing
international effort by the United States, Canada and Argentina to
change the chemical practices of Argentine farmers.

The new satellite tracking of the migration helps explain why none of
the bands returned from dead birds in Argentina were from Sacramento
birds. It also is consistent with findings by James Estep, a private
consulting biologist in Sacramento, that the vast majority of Yolo
County birds were coming back every year.

"It was kind of nice to find out, at least so far, that we may not be
subject to those particular problems that most of the rest of the
species is subjected to in Argentina," Estep said. "Of course, we may
have our own in Mexico."

Bradbury, Estep and raptor specialist Richard Anderson of the California
Energy Commission will travel to southern Mexico next month to take
their first look at the wintering habitat of the Central Valley
Swainson's hawks.

The birds will arrive back in their Central Valley nesting areas in
March. The female often arrives first, researchers say, returning to the
exact nesting site to await her faithful mate.

The birds are secretive and ferociously territorial during this time,
but after the fledging of the young in the summer, the Swainson's hawks
become social, flocking in large groups and feeding in preparation for
the fall migration.

Rob Saecker
Olympia