Caterpillars, forest birds and trees … how they fit

I recently heard a lecture by Jim McCormac, a biologist with the Ohio DNR. He argued that caterpillars, forest birds and trees were dependent upon each other for survival.

Each spring, deciduous woods are filled with both migratory birds moving north and residential breeding birds.

Let’s examine how small songbirds such as warblers, vireos and flycatchers act as keystone species in forest ecosystems.
You may want to grab a calculator to follow along. As I proceed, I will make a series of assumptions.

All will be conservative, which will make the outcome even more impressive. For example, we’ll assume that most of the caterpillars under consideration are larval moths, but of course birds devour fleshy invertebrates of any sort — flies, gnats, spiders, etc.

Foraging birds

If we surveyed a 100-acre deciduous woodlot 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.

They especially prefer the young, tender leaves that unfurl in May.

On average, a songbird uses about an acre of foraging habitat each day. That means that our 100-acre woodlot will be home to at least 100 individual birds of a variety of species. The birds that love these caterpillars are the ones that stay high in the treetops and are difficult to see even with binoculars.

A few minutes spent watching birds in the treetops will convince any birder that forest birds spend most of their time feeding. They must recover energy lost during migration and acquire energy needed for breeding.

Since it’s tough to get a good look into the treetops to confirm that they are alive with caterpillars and other insects, next time a tree or large branch comes down in a storm, check out the leaves.

You will find myriad tiny inchworm-like caterpillars — perfect bird food.

Calculating the numbers

Now, let’s assume that each bird eats one caterpillar each minute all day long. That’s one caterpillar per minute X 60 minutes per hour X 12 hours (7 a.m. to 7 p.m.) = 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. They are also eating lots of other insects, spiders, and other invertebrates.

Because land area can be difficult to envision, an acre is a square about 209 feet on a side. Thinking bigger, one square mile (a square of one mile on each side) is 640 acres.

So let’s extrapolate again. If every bird eats 720 caterpillars each day, then 640 birds occupying 640 acres would eat 460,800 caterpillars (720 caterpillars X 640 acres). Now double that number to 921,600 caterpillars, because we assume that each of our original 100 birds has a mate.

So forest birds eat close to one million caterpillars on every square mile of forest every day. To make these numbers even more impressive, remember that the nesting season is underway. So let’s add caterpillars fed to nestlings to the mix.

In addition to what adults eat, they feed nestlings every few minutes all day long during the ten to twelve-day nestling period. And on each visit to the nest parents usually bring several caterpillars to feed the chicks.

During this time, the total number of caterpillars consumed by forest birds is a multiple of average clutch size (three or four chicks per nest). Furthermore, most of these species raise two broods per year.

Bottom line

In May and June forest birds remove millions of caterpillars from every square mile of deciduous forest every day. Forest birds are nature’s perfect pest control agents. And they’re free. It’s just another reason to enjoy and appreciate wild birds.

At this point, it doesn’t take a genius to see that if a catastrophic event wiped out the birds of a forest, there would be nothing to control the caterpillars that eat the leaves of the trees. And without leaves, trees die.

So if forest birds disappear, the forest itself will die just a few years later. McCormac was right. Caterpillars, forest birds, and trees are indeed interconnected.