Farming Inside Forests Hurts Bird Communities More Than Timber Harvesting, Study Suggests

COLUMBUS, Ohio - Farming in and around forests hurts bird populations more than does timber harvesting, according to a study by a researcher at Ohio State University. The study suggests that farming may make bird nests more vulnerable to predation by squirrels and other animals.

Amanda D. Rodewald, assistant professor of wildlife ecology in the School of Natural Resources, also found that forests with agricultural clearings in them had a lower diversity of bird species and fewer birds than forests in which pockets of tree cover had been cleared by timber harvesting.

"A farm patch in the forest may be more damaging to birds than a clearing left by just the felling of trees," said Rodewald, who did the study in collaboration with Richard H. Yahner, a professor of wildlife ecology at Penn State University. The researchers published their work in a recent issue of the journal Ecology.

Rodewald said the most likely reason for the adverse impact of agriculture on nesting success was that the farm patches were somehow helping nest predators to thrive.

Rodewald said the results of the study could help make more informed decisions on strategies for conservation and forest management.

"Some parks and forests will allow small agricultural openings within them. Those are the types of management policies that we need to start reconsidering," Rodewald said. "It's not enough to say there are a lot of forests, so the birds in them will be fine. We have to look at land uses right next to the forested zone."

Rodewald and Yahner looked at a total of 10 forests in their study, all of them in central Pennsylvania. Five had farm patches in them while the remaining five had clearings left by timber harvesting. The forests were roughly equal in size.

The researchers surveyed bird populations inside the forests during the breeding seasons between 1997 and 1999. Much of this work involved identifying individual nests and observing the activity of birds inside them.

"The first time we looked into a nest, we would make notes about the number of eggs inside," Rodewald said. "In subsequent observations, we would either find that the eggs had hatched successfully and the chicks were developing feathers - getting ready to fly out - or that the nestlings were gone and the nest had been destroyed. In the second case, we knew that the nest had most likely been visited by a predator."

Rodewald said the observations were made carefully - most of the time through binoculars - so as not to disturb the nests. For nests hidden deep within the foliage of tall trees, the researchers managed to look inside the nests using mirrors tied to the ends of long poles. Before making their observations, they would usually wait until the female left the nest to bring back food.

After accounting for the differences in the amount of disturbance within each forest, Rodewald and her colleague found that nesting success in forested landscapes disturbed by farming was about two and a half times lower than in those disturbed by timber harvesting.

Rodewald said the most likely reason for the adverse impact of agriculture on nesting success was that the farm patches were somehow helping nest predators to thrive.

"Maybe the predators are getting supplemental food from these agricultural openings," Rodewald said. "Maybe it just helps them survive the winter, so you end up with higher populations of predators each year."

To compare the diversity and abundance of birds in the two kinds of forests, the researchers studied 34 different landscapes - half of them disturbed by agriculture and the other half by timber harvesting. This sample included the 10 forests that Rodewald and her colleague looked at in their nesting success study.

From the larger survey, the researchers found that the number of bird species in forests disturbed by farming was lower. On average, the abundance of birds in these forests - measured by the population of each species - was smaller as well.

Another interesting finding was that the nature of disturbance - farming or timber harvesting - seemed to matter more than the size of the clearing. "Forests with small agricultural clearings were worse off than forests that had lost relatively larger areas to timber harvesting," Rodewald said.

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Contact: Amanda Rodewald, (614) 247-6099; Rodewald.1@osu.edu

Written by Yudhijit Bhattacharjee, (614) 292-8456; Bhattacharjee.5@osu.edu