For more than a decade, biologist Brian Woodbridge
watched hundreds of Swainson’s hawks raise their young in the
fields of Butte Valley in northern California. Each fall, the birds
headed south, but Woodbridge spotted a strange
pattern.
“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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