Black-capped chickadees avoid feeding on caterpillars that have eaten leaves with high levels of carbon dioxide, a finding that researchers say could mean more gypsy moth caterpillars and fewer chickadees in Rhode Island.

01:47 AM EST on Saturday, March 20, 2004

BY EDWARD ORTIZ
Journal Environment Writer

With the onset of spring, songbirds are returning to feeders and nests as scientists are taking a close look at whether the birds are being affected by global warming.

One recent study, conducted at the University of Rhode Island, has found strong evidence that the black-capped chickadee, a common songbird in New England, avoids eating insects that have ingested leaves with high levels of carbon dioxide (CO2).

The study offers insight into the far-ranging effects of climate change. To date, few studies have looked further than the effects of high CO2 levels on insects. As a result, the URI study becomes one of the first to look at the effects of elevated CO2 levels one step further along the food chain.

CO2, a pollutant emitted into the atmosphere primarily by power plants and automobiles, is a major contributor to global warming.

Journal photo / Bob Thayer A black-capped chickadee takes flight at the Caratunk Wildlife Refuge, in Seekonk. A study by the University of Rhode Island shows that this type of bird cannot easily digest insects with high concentrations of carbon dioxide.

Although the research breaks new ground, studies looking into the effects of climate change on bird populations are many and well documented.

Scientists have already established that bird migrations are changing because increased temperatures have altered food supplies. In a recent article published in the periodical Nature, researchers reported that a dozen British bird species have shifted their migrating ranges an average of 12 miles over the last 20 years. In the United States, the migrating range of the golden-winged warbler has moved 100 miles north over the past two decades, and scientists expect that bird species rarely seen in New England will start appearing if climate-change rates continue.

The goal of the URI study was to establish whether a group of 30 chickadees caught in Kingston could detect the presence of two compounds -- tannins and phenolics -- in 30,000 caterpillars fed leaves from trees that produce the compounds when subjected to high levels of CO2.

The genesis for the study came after McWilliams talked shop with entomologist Richard Lindroth about Lindroth's research at the University of Wisconsin. That research showed that gypsy moth caterpillars grew into much smaller adults after they ate significant amounts of aspen, birch, and oak trees high in CO2 concentrations.

"He was trying to make the case for large effects that compounds have on insects," said McWilliams.

McWilliams immediately began to think about birds, and whether they would prey on caterpillars whose diets were affected by global warming.

To find the answer,McWilliams imported aspen trees that were grown in Wisconsin for research on gypsy moth caterpillars. The trees produced the same balance of tannins and phenolics representative of the CO2 levels found in a North American forest.

McWilliams said the leaves were an accurate representation of the elevated CO2 levels that are found in New England.

McWilliams and Muller monitored the chickadees to see if they showed any preference when given a choice of caterpillars that ingested leaves that had high concentrations of CO2 or those with lower concentrations. After three days, the birds showed strong preferences against caterpillars with high levels of the compounds.

The birds cannot easily digest the compounds suggesting that chickadees can either sense the makeup of their food or memorize the digestive effects of their food, said Muller.

"Tannins affect the digestion of chickadees," she said. "They reduce the digestibility of proteins and that reduces the nutrients available to them."

High tannin levels force the birds to eat more and expend more energy to detoxify themselves, Muller said.

Black-capped chickadees were chosen because the species is intelligent and has a good memory compared to other songbirds, said McWilliams. Memory is crucial to the food gathering activities of the songbird, as they are fond of storing food in tree bark for as much as 28 days.

McWilliams said that it is possible that the effects of climate change on the composition of gypsy moth caterpillars will make songbirds switch from one feeding locale to another where CO2 concentrations are lower. Food shortages are not out of the question as result of the high compound levels spurred by global warming, he said. During the spring and summer breeding season, chickadees eat a diet composed 80 percent of animal protein such as caterpillars and spiders.

McWilliams offered the caveat that chickadees avoiding gypsy moth caterpillars as a food source does not necessarily translate into a caterpillar population boom.

However, he did say that if all groups of songbirds lay off the caterpillars, then forest defoliation is likely.

Jeff Price, director of climate change impact studies at the national nonprofit American Bird Conservatory, in Virginia, cautions that the study, though highly important, is only indicative of the behavior of one species, and not all songbirds.

However, Price agrees with the contention that global warming has affected the behavior of bird populations,including the black-capped chickadee. Price developed a model combining temperature and rainfall variables that mimic current global warming rates.

That model helped him establish that within 75 years, the population of black-capped chickadees would have migrated north and no longer be found in Rhode Island. Currently, the black-capped chickadee is found as far south as North Carolina.

Edward Ortiz has a fellowship with the Metcalf Institute for Marine and Environmental Reporting. He can be reached at eortiz [at] projo.com

DIGITAL EXTRA: Browse a report on the impact of global warming on birds nationwide, find local birding links and listen to the call of the black-capped chickadee:

http://projo.com/extra/2004/birding/