College of LAS « Illinois

Animal Biology

Birds Are Natural Insect Controls in Tropical Forests

Nature lovers now have another argument to use when advocating for the protection of bird habitat.

Research conducted high in the treetops of a Panamanian forest, showed that birds, especially those found only in the tropics, are a natural form of insect control.

The finding was a mild surprise to the scientists, who knew that birds protect crops and temperate forests from being devoured by insects, but they had theorized that the rich diversity of life in tropical forests diffused any significant contributions by birds.

"We found that birds were indirectly defending trees by consuming their herbivore pests," says Sunshine A. Van Bael, a doctoral student in animal biology and a research fellow at the Smithsonian Tropical Research Institute. "This is important because we need our trees, especially in the tropics, to be healthy. They provide us with oxygen and sequester carbon emissions, protecting us from global warming. Understanding how animals interact with trees will help us keep the forests healthy."
Worldwide, but especially in neotropical forests, bird populations are declining as forests are cleared for ranching, farming, or housing.

During the rainy months of May to December, the number of arthropods chomping on trees is 90 percent higher than in the dry season. Native tanagers, including the blue-gray, crimson-backed, plain-colored and white-shouldered, and native vireos, particularly the lesser greenlet, dined extensively on caterpillars easily visible on the tops of leaves.

The 31 species of birds that Van Beal observed ate enough to drastically reduce damage to leaves in comparison to damage done to leaves in specially built exclosures where access by birds was limited. Where the birds could not reach, the densities of leaf-eating insects were much higher. By the end of the rainy season, the damage to foliage in these restricted areas was 86 percent higher than in the areas patrolled by birds. Migratory temperate-zone birds, including several warblers, also gobbled insects off the leaves in the late rainy season. In the dry season, when insects were scarcer, the birds switched to eating trees' fruit.

Van Bael made her observations from a small gondola that was suspended above the forest canopy by a construction crane.

October 2003