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CHRIS D. THOMAS AND JACK J. LENNON Nature, May 20, 1999
We have analysed the breeding distributions of British birds
over a 20-year period. After controlling for overall population
expansions and retractions, we find that the northern margins of
many species have moved further north by an average of 18.9 km
during this time. This general northward shift took place during
a period of climatic warming, which we propose might be causally
News report from Discovery:
Study: Birds Nest Farther North
Climate warming could be pushing birds farther north to breed,
a new study suggests.
Ecologists Chris D. Thomas and Jack J. Lennon of the University
of Leeds in England analyzed the distributions of nesting birds
in Britain over 20 years, roughly between 1970 and 1990. The
northern boundaries of the ranges of many species appear to have
moved northward, on average by 11.7 miles, the scientists report
in this week's journal Nature. Previous studies have suggested
that changes in climate are spurring many organisms -- including
butterflies, birds, amphibians, and plants -- to head to cooler
regions either by moving northward, or climbing mountains.
But many of these studies were unable to prove that the shifts
were caused by true migrations rather than by changes in total
area occupied, as occurs when populations grow or shrink.
Using data from two atlases of breeding birds in Great Britain,
Thomas and Lennon measured changes in the boundaries of
areas of more than 100 bird species and compared them to
in breeding ground area.
"This sort of creativity in analyzing what little data we have
is extremely important," says ecologist Camille Parmesan of the
University of Texas in Austin.
Thomas and Lennon estimate that, on average, the northern edges
of species that breed in southern Britain have moved almost 12.4
miles north in 20 years. Surprisingly, they did not observe a
corresponding shift in the southern borders of northerly species.
For now, nobody is exactly sure why.
The researchers blame climate warming for the northward shift
because the shift coincided with a period of warming, because
other studies indicate that changes in temperature affect bird
reproduction, and because the distribution of bird species in
Britain is correlated with temperature.
"I'm surprised the (northward) changes were so slight," says
ornithologist Jeff Price of the American Bird Conservancy in
Colorado, who is conducting similar studies in North America.
"On its own, it isn't really strong evidence for climate warming
shift," Price says." He says he's measured a 45.3-mile northward
shift in a warbler population over 24 years, for example.
"It's when you put all the studies together that you start
building a picture of what's happening," says Parmesan. "And that
picture is increasingly saying that very small changes in climate
make quite large changes in natural systems."
By Marina Chicurel, Discovery
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