Trees need caterpillars and birds

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I recently heard a lecture by Jim McCormac, a biologist with Ohio Department of Natural Resources. He argued that caterpillars, forest birds and trees are dependent upon each other for survival.

Each spring, deciduous woods are filled with migratory birds moving north and residential breeding birds. Let's examine how small songbirds such as warblers, vireos and flycatchers function as keystone species in forest ecosystems.

If we surveyed a 100-acre deciduous woodlot in Western Pennsylvania today, we would find at least 20 species of songbirds foraging in the treetops. These birds feed almost continuously on the tiny fleshy caterpillars that eat the leaves in the tallest trees.

On average, a songbird uses about an acre of foraging habitat each day. That means our 100-acre woodlot will be home to at least 200 individual birds (assuming each one has a mate) of a variety of species.

Let's do the math. Assume that each bird eats one caterpillar per minute all day long. That's one caterpillar per minute, times 60 minutes per hour, times 12 hours per day (7 a.m. to 7 p.m.). That equals 720 caterpillars per bird per day per acre.

Extrapolating this to our 100-acre woodlot, our 100 birds are eating 72,000 caterpillars per day (in addition to many other insects, spiders and other invertebrates).

Now let's extrapolate again to 1 square mile of forest, because a square mile is easier to visualize than 100 acres. If every bird eats 720 caterpillars each day, then 1,280 birds occupying 640 acres (1 square mile) eat 921,600 caterpillars. So forest birds consume close to 1 million caterpillars on every square mile of forest every day.

Forest birds are nature's perfect pest control agents. It's just another reason to enjoy and appreciate wild birds.

If a catastrophic event wiped out the birds of a forest, there would be nothing to control the caterpillars that eat the tree leaves. And without leaves, trees cannot survive. So if forest birds were to disappear, the forest itself would die just a few years later. McCormac was right.

Biologist, author, and broadcaster Scott Shalaway can be heard 8-10 a.m. Saturdays on 1370 WVLY-AM (Wheeling) and online at He can be reached at, and 2222 Fish Ridge Road, Cameron, W.Va., 26033.

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