“I noticed that some years a lot more adults returned from migration than others,” he says. “That really got me wondering.”
In 1994, Woodbridge tried a modest experiment. He fitted two hawks with radio transmitters and followed their journey using weather satellites. The hawks flew south through Mexico, El Salvador, Nicaragua and Bolivia to the Argentine pampas.
Woodbridge followed them to Argentina and found an impressive congregation of Swainson’s hawks feasting on grasshoppers. But the hawks were dying by the thousands. Woodbridge and his colleagues found 700 corpses under a single roost, including birds that had been banded by biologists in Saskatchewan and Utah as well as the Butte Valley.
Investigating, he found that the grasshoppers were laced with a pesticide, monocrotophos, that is not sold in the United States, but is widely used by Argentine farmers to kill the insects.
Woodbridge is now working with Argentine and Canadian scientists to protect wintering hawks by helping farmers find alternative means of pest control. And while mortalities were much lower last year, Woodbridge is not claiming victory. Farmers were willing to lay off pesticides because grasshopper numbers were low.
Woodbridge adds that farming and development on the West’s open grasslands are squeezing Swainson’s hawks out of their summer habitat. “We can’t just point our fingers down south,” he says. “We have our own problems up here.”
* Sharon Levy
This article appeared in the print edition of the magazine with the headline Crossing borders to save hawks.
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