Excerpted from Living on the Wind: Across the Hemisphere with Migratory Birds

In this excerpt from Living on the Wind, author Scott Weidensaul details the research conducted at a ranch on the pampas of Argentina, the winter home of the Swainson's hawk, a lovely migrant from the prairies of North America known as the aguilucho to Argentines. In recent years, farmers using a powerful pesticide called monocrotophos had accidentally poisoned tens of thousands of the hawks, leading to a frantic, international effort to learn more about their ecology in Argentina, and to stop the slaughter.

At 3:45 a.m., the Argentine sky is awash with stars, the Southern Milky Way like confectioner's sugar splashed across the middle, and Orion standing on his head in the north. But I am blind to the spectacle, barely able to drag myself out of my sleeping bag and dress. We are caught in a collision between biology and culture. Agustmn and his family, like most Argentines, keep a very European schedule, and the evening meal isn't served until ten thirty or eleven o'clock at night; no one gets to bed before midnight. Yet the hawks rise before the sun, and at this time of year, the sun rises very early indeed. We Americans are falling further and further behind in our sleep each night.

The bleached skull of a Swainson's hawk lies
among the leaves in a woodlot at Estancia La
Chanilao. In 1995 and '96, tens of thousands
of these migratory hawks were killed in
Argentina by the pesticide monocrotophos,
sparking an international effort to save them.

In a cramped room beside the old ranch bunkhouse, a bleary-eyed Mike Goldstein snaps on a battery-powered lantern and growls,"Let's get moving. It'll be light in an hour." Twenty-nine years old, bearded, with a shock of dark hair framing a high forehead, Goldstein is a doctoral candidate at Clemson, and his job at Chanilao is to document whether the hawks are being exposed to chemicals  not only monocrotophos, but a range of potential toxins.

Swainson's hawks are among the
long-distance champions of North America
landbirds, traveling up to 8,000 miles
each autumn from the prairies of the U.S.
and Canada to the grasslands of
Argentina, where they feed mostly
on grasshoppers and other insects.

Mike grabs two brown lab mice by the tails and pops them into a flat, wire-mesh cage about a foot square and two inches deep, covered on top with dozens of small nooses made of fishing line. This is a bal-chatri trap, an ancient Asian falconer's device once made of loosely woven baskets and horsehair loops. If a hawk tries to grab a mouse, its toes will become harmlessly ensnared in the nooses. We will place a dozen of them in a field of newly sprouted sorghum alongside the monte, but with a thoroughly modern twist. Beside each, we will bury a small, cigarette-pack-sized radio transmitter, its whip antenna poking up through the dusty soil, and clipped to the trap by a thin cord. Any movement of a captured hawk will activate the transmitter, allowing us to monitor many traps over a wide area of the ranch.

It's cool, with a chilly breeze. I walk through the dark monte, listening to the carpet-slap wingbeats of frightened hawks overhead. With me are two of Agustmn's teenage daughters from his first marriage  Candi, fourteen, and Agustmna, seventeen, slim and dark like their father, both very pretty and both very capable hawk-catchers. They show me how to arrange the traps in a huge grid across the field, mounding up the soil to make a little platform for each, burying the transmitter and a small, dumbbell-shaped weight nearby. As we finish, the sky is turning purple and the Milky Way fades to nothing.

Then there is a little time for mate or coffee back at the bunkhouse, listening to the throaty coos of spot-winged pigeons. Raptors start flying out of the woods in the half-light, but they are chimangos, not Swainson's hawks. Chimangos are caracaras, a group of scavengers related to falcons, although they look nothing like those sleek, aerial hunters. The size of crows, with long, naked legs, the dingy brown chimangos are ubiquitous in this part of the pampas; it is hard to look in any direction without seeing a few sitting on fenceposts, rowing across the sky or squabbling over a roadkilled lizard.