ScienceDaily, October 31, 2011
Birds are
getting bigger in central California, and that was a big surprise for Rae
Goodman and her colleagues. Goodman uncovered the trend while working as a
graduate student for Associate Professor of Biology Gretchen LeBuhn, analyzing
data from thousands of birds caught and released each year at two sites near
San Francisco Bay and Point Reyes National Seashore.
The SF State
scientists found that birds' wings have grown longer and birds are increasing
in mass over the last 27 to 40 years.
What's making the
birds bigger? The researchers think that the trend is due to climate change,
but their findings put a twist in the usual thinking about climate change and
body size. A well-known ecological rule, called Bergmann's Rule, states that
animals tend to be larger at higher latitudes. One reason for this rule might
be that larger animals conserve body heat better, allowing them to thrive in
the generally colder climate of higher latitudes.
Under this
reasoning, some scientists have predicted that animals would get smaller as
Earth has warmed up over the past 100 years. But the study, published in the
journal Global Change Biology,
suggests that the connection may not be so simple.
Climate change may
affect body size in a variety of ways, they note in their paper. For instance,
birds might get bigger as they store more fat to ride out severe weather
events, which are expected to be more common under global climate change.
Climate change could also alter a region's plant growth, which may eventually
lead to changes in a bird's diet that affect its size.
LeBuhn said she was
"completely surprised" to find that the central California birds were
growing larger over time. "It's one of those moments where you ask,
'what's happening here?'" The results were so unexpected, she said, that
the findings made them take a step back and look more closely at how climate
change could influence body size.
The bird data come
from two long-term "banding stations" in central California, where a
wide variety of birds are captured, banded about the leg with an identification
tag, and weighed and measured before being released. Many of the same birds
were captured each year, allowing the researchers at the sites to build up a
unique database that could be used to track changes among the birds over
several decades.
The researchers
used data from 14,735 individual birds collected from 1971 to 2010 at the
Palomarin Field Station, near the southern end of the Point Reyes National
Seashore, by researchers from PRBO Conservation Science. Their study also
included data on 18,052 birds collected between 1983 and 2009, from the Coyote
Creek Field Station at the southern end of the San Francisco Bay by the San
Francisco Bay Bird Observatory.
"At the time I
started my research, a few studies had looked at body size changes in a few
species in Europe and the Middle East, but no one had examined bird body size
changes in North America," said Goodman, who graduated from SF State in
2010 and now teaches biology and environmental science at San Francisco's
Jewish Community High School of the Bay.
"We had the
good fortune to find an unexpected result -- a gem in research science,"
she added. "But we were then left with the puzzle of figuring out what was
going on."
After testing and
discarding a number of other explanations, Goodman and her colleagues were
confident that climate change was behind the longer wings and bigger bodies in
most of the birds. The birds may be responding to climate-related changes in
plant growth or increased climate variability in central California, the
researchers suggest in the paper.
The findings offer
a glimpse at the potent effects of climate change across a wide range of
species, LeBuhn said. "Even over a pretty short period of time, we've
documented changes in important traits like body size, where we don't expect to
see much flexibility."
"But
in some ways," she added, "it gave me a little more hope that these
birds are able to respond -- hopefully in time -- to changes in climate."
Story Source:
The above story is reprinted from materials provided by San Francisco State University.Note: Materials may be edited for content and length. For further information, please contact the source cited above.
Journal Reference:
- Rae E. Goodman, Gretchen Lebuhn, Nathaniel E. Seavy, Thomas Gardali, Jill D. Bluso-Demers. Avian body size changes and climate change: warming or increasing variability? Global Change Biology, 2011; DOI:10.1111/j.1365-2486.2011.02538.x
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