Back from the Boneyard

Saving the Swainson's Hawk: Part 3
By Scott Weidensaul
Page 3 of 4   |  
field of sunflowers
In the past 15 years, much of the pampas has been converted from pasture to row crops, especially sunflowers, prompting farmers to rely more heavily on dangerously toxic chemicals.

Meanwhile, back at the ranch (I've always wanted to write that), an assembly line is in operation, processing the dozen and a half birds we catch through the morning. While one person holds a hawk, another carefully washes its feet and legs with ethanol and catches the rinse, which will be analyzed back at Clemson for contaminants that might have been picked up walking in sprayed fields; the freshly cleaned toes are as yellow as dandelions. Tiny snippets of feathers are clipped from the belly, wings, and back, and slipped into plastic specimen bags; these, too, will be tested for chemicals. After banding, another two-person team draws two small vials of blood from each hawk for analysis, then the bird is handed to another set of researchers who weigh, age, and measure it. Finally, an hour after it was captured and no doubt feeling as poked and violated as an alien abductee the hawk is released in the yard of the ranch. As a final indignity, the pugnacious flycatchers that nest in the tall tree by the bunkhouse harass each hawk out of sight, strafing it until it clears the edge of the woods.

Goldstein's toxicological work is just one facet of the research going on at Chanilao. Every few days, the team receives, via computer from Idaho, the latest locations of two dozen Swainson's hawks fitted with satellite transmitters; the coordinates show that many of them stopped well to the north, where no one has been looking for hawks. (Several members of the team later drive into the northern pampas, confirming the presence of large flocks there.) Because even the simplest questions about how the hawks live on the pampas remain unanswered, another twenty have been fitted with standard radio transmitters and shadowed by a team of Argentines headed by Sonia Canavelli. What kind of habitat do they choose for roosting? When and where do they eat? Are the huge flocks an accident a result of patchy food supplies, perhaps or are the hawks somehow dependent on dense aggregations for their survival, as were the passenger pigeons to which they are often compared? How sensitive are they to human disturbance? Do the flocks wander widely, or do they remain in the same general area for long periods of time? Is there segregation by age, since most of the birds seen in La Pampa are adults, while flocks of mostly juveniles have been seen hundreds of kilometers to the east? There are far more questions than answers at this point, and any one of them could be crucial to the long-term survival of the Swainson's hawk.

In that sense, the aguiluchos are perfect examples of the challenges facing conservation biologists when dealing with migratory birds, for the most fundamental information is often lacking. Take food. There are mountains of studies on what Swainson's hawks eat in North America, but only guesswork and limited observation in Argentina. It is assumed they feed heavily on grasshoppers; after all, that's what got them in trouble with pesticides in the first place. But there is one report, from the eastern pampas south of Buenos Aires, of flocks of juvenile birds feeding heavily on swarms of migratory dragonflies. It may be that the hawks here are catching dragonflies as well, on the wing during the hot midday hours when they rise beyond the limits of vision. If true, that would be reassuring, since the grasshoppers are under fierce assault by farmers.

To answer the question of diet, we must collect regurgitated food pellets, or castings, the indigestible remains of the bird's meal that are regurgitated once or twice a day. The ground beneath the roost trees is a mess splashed with white guano and littered with freshly molted wing and tail feathers. Scattered liberally beneath the trees are dry, pinkish lumps the size of walnuts, many of them disintegrating into granular piles. Most of the hawk pellets I've seen over the years were made up of packed fur and bone fragments, but from what I can see, these are comprised entirely of the chitonous bits of grasshoppers' legs, mandibles and pieces of the outer shell, graphic evidence of the aguiluchos' insectivorous appetite. Someone not me, thank God will have the thankless job of picking the pellets apart under a dissecting microscope and identifying all the fragments, one by brain-numbing one.

Published: 28 Apr 2002 | Last Updated: 15 Sep 2010
Details mentioned in this article were accurate at the time of publication


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