CO2 Increases Impact Songbird Feeding
- Media Contact: Todd McLeish 401-874-7892
Global warming may cause songbirds
to avoid certain foods
URI student researcher: Chickadees avoid caterpillars
that eat leaves exposed to high levels of CO2
KINGSTON, R.I. -- January 26, 2004 -- In yet another example of the
far-reaching impact of global warming, a University of Rhode Island
student found evidence that suggests some songbirds may avoid eating
insects that consume leaves exposed to high levels of carbon dioxide.
URI senior Martina M�ller of Kingston, working in cooperation with
Associate Professor Scott McWilliams, Ph.D. candidate David Podlesak and
colleagues at the University of Wisconsin, studied the food preferences
exhibited by black-capped chickadees.
"When plants are grown in conditions of higher carbon dioxide, they
produce increased levels of several secondary compounds -- tannins and
phenolics -- that they use to defend against herbivory," said the
23-year-old wildlife conservation and biology major. "Those secondary
compounds are absorbed by gypsy moth caterpillars that feed on the
plant�s leaves, which other researchers have found reduces the
caterpillar�s growth rates. We wanted to see if the chickadees can detect
the secondary compounds in the caterpillars and if they have preferences
for caterpillars that fed on different types of leaves."
Using chickadees captured in Kingston and acclimated for three days,
M�ller and McWilliams fed the birds a choice of caterpillars that were
high in tannins or phenolics and other caterpillars low in those
"It was clear that the birds could tell the difference between the
different caterpillars and they had strong preferences," M�ller said.
"They�re intelligent birds with a keen capacity to learn."
While the birds showed a distinct preference for caterpillars low in
tannins and phenolics, they also showed a preference for foods they had
eaten previously. "Previous experience does affect their preferences,"
So what does all this mean? According to McWilliams, it could mean a
great deal in a world that is growing warmer due to increasing levels of
carbon dioxide in the atmosphere. "These results provide a much more
complete and realistic picture of how elevated atmospheric CO2 might
affect ecological systems."
Since increased carbon dioxide leads to elevated levels of secondary
compounds in plant leaves and decreased growth rates of caterpillars that
eat those leaves, McWilliams said "birds that primarily eat herbivorous
insects like caterpillars may find themselves without enough to eat as
atmospheric CO2 levels increase. In short, chemicals in the caterpillar�s
food influences the likelihood of predation by birds."
In addition, he said that if birds avoid feeding on gypsy moth
caterpillars, for instance, an uncontrolled population of the
caterpillars could result in more severe forest defoliation.
McWilliams also sees a connection between M�ller�s results and the
current mad cow disease concerns in the U.S. "We know that mad cow
disease can be transmitted to humans if we eat beef from cows that have
eaten feed with the disease. So to safeguard our beef, we feed cows food
that does not contain the disease. Birds seem to pay attention to this
same rule: know what the food you are eating has eaten, because it can
affect your health. In the case of birds, however, they seem to be one
step ahead of us in that they are able to detect the secondary compounds
in the food and change their feeding behavior accordingly."
Funding for this research was provided by the National Science
Foundation, the URI Agricultural Experiment Station, and the URI Coastal
Fellows program, a unique program designed to involve undergraduate
students in addressing current environmental problems. The caterpillars
and aspen leaves used in the project were provided by Professor Richard
Lindroth and Ph.D. candidate Jack Donaldson at the University of
For Further Information: Scott McWilliams 401-874-7531
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