Chuck Tague

(How are you celebrating Earth Day?)

    We meet some every spring.  I call them leaf-cursers: birders, hopefully novices, who swear at the trees.  “*#@*!” they whine as they massage the kink in their neck.  “I wait all year for warblers.  Soon as they get here the *#@*! leaves pop out and I can’t find the birds.”

    I just grin.  I once felt compelled to straighten them out but now I let them go on their naive, egocentric way.  I figure after two or three springs, they’ll make the connection.  (It took me six.)

    Leaf-cursers are unaware they’ve stumbled onto an astute observation that evades most people.  The accepted view is that the vernal greening is a slow, gradual process, like the germination and development of marigold seeds in window pots.

    In fact, each spring the forest comes alive with an earthshaking explosion of energy; energy generated the previous summer and stored in roots and twig buds.  The leaf-out of the trees is rapid and well coordinated.  In a forest with hundreds of trees per acre, each tree pumps gallons of water from the ground to the tips of the highest twigs.  This hydraulic pressure forces minerals and organic chemicals, as well as root-stored sugars, to the bursting leaves.  The new green leaves immediately begin the process of “photosynthesis.”  The switch from apparent dormancy to vitality is literally overnight.

    The annual greening advances northward with the lengthening daylight.  It proceeds with predictable precision.  Warblers and other songbirds move north with the greening.

    This explosion of life not only sets the timing of songbird migration, it provides the means.  Any sudden availability of food attracts hordes of opportunistic feeders.

    Zoological geographers refer to tropical America as the Neotropics.  Warblers, and other songbirds that migrate north from the Neotropics, move in waves.  These waves of songbirds (nicknamed neotrops) are always near the edge of the greening.  The waves are fueled by the forest’s release of energy and nutrients.

    Why do neotrops make this long, treacherous journey?  They do it because the reproductive advantages in the temperate forest in spring outweigh the hazards of the journey.

    The release of the forest’s life-forces is followed by a period of intense production.  The trees make food for their immediate sustenance and growth while they build up an excess to produce flowers, pollen, fruit and seeds.  The trees also store food and energy to sustain them through the long winter and fuel next spring’s awakening.

It’s the trees that produce resources for the birds’ annual molt; the nourishment that fattens them and their offspring for the long return to the Neotropics.

    Superficially, the tree/bird connection is straight forward.  There are two catches, however.  First, the trees are extremely protective of their fresh leaves.  Second, leaves are bulky, heavy and difficult to digest; not an appropriate food for the high-energy demands of small, flying animals.

    On a seemingly tranquil late April morning, a birder’s senses are bombarded: a symphony of bird song, the fragrance of apple blossoms, buzzing bees, scurrying small mammals, squealing raptors, drifting butterflies, colorful wildflowers, sunning snakes and trilling toads.  Eventually, the leaf-cursers, the tunnel-visioned warbler-watchers and the compulsive listers realize there’s more going on than nerds squinting through leaves for a glimpse of feathers.

    Even seasoned naturalists are distracted by a tanager or a singing thrush.  We forget we’re watching a conflict of epic proportions.  Songbirds play a small part in the battle but their presence, or absence, tips the balance of power.  Birders are nothing more than inconsequential bystanders.

   Leaves are most vulnerable shortly after leaf-out.  The trees do not yet have all their defenses in place.  Plants defend their leaves with tough outer coverings, sharp spines, irritating hairs, a composition that defies digestion and infusions of toxic chemicals.  But plants are never invincible.  Their security can, and will, be breeched.

    As the spring leaves emerge, hordes of leaf munchers attack.  Some come singly, quietly under the cover of darkness.  Others attack in well-coordinated battalions.  The vast majority of the attackers are vermiform herbivores from the insect order Lepidoptera: larvae of moths and butterflies.  We call them caterpillars. 

    Caterpillars are by far the most sophisticated leaf-predators.  They represent a single stage in the life of a butterfly or moth, the stage between egg and pupa.  The pupa, of course, transforms the insect from a sluggish, foliage-eater to a flying, reproductive adult.  A caterpillar consumes huge quantities in preparation for metamorphosis and becomes a nutritious bundle of proteins and calcium.  Ecologically, the caterpillar population is the fulcrum that supports the forest’s delicate balance.  To songbirds, caterpillars are their conduit to the forest’s vast store of food.

    To a birder caterpillars are a mystery.  Except for Tent Caterpillars and tiny inchworms that dangle from silk threads, spring caterpillars are a force as invisible as the wind.  We see their damage on leaves.  Occasionally we see a bird with a juicy green one in its beak.  Caterpillars have a powerful presence.  Their biomass is huge.  We just can’t see them.

    This is not a coincidence.  The survival of most spring caterpillars depends on their ability to escape detection.  Simultaneous with the caterpillars’ attack on the emerging leaves, hordes of worm-eaters descend on the forest.  The army of vermivores is not just songbirds, but spiders, ants, rodents and parasitic wasps.

    Spring caterpillars are the vanguard, leading the way in a battle that continues through fall.  They don’t have time to develop intricate defensives.  Instead they use camouflage.  Most feed at night.  During the day the caterpillars assume the colors, patterns and shapes of objects in

the forest. 

They impersonate leaves,

leaf-stems, twigs, lichens

or flakes of bark.

    Their disguises are diabolically convincing.  Hordes of sharp-eyed songbirds -- kinglets, vireos, warblers, tanagers, buntings and grosbeaks -- scrutinize every leaf, bud and twig.  These waves of birds have a search image fine-tuned by months and years of experience and millennia of natural selection, but they probably miss more caterpillars than they find.  Enough caterpillars evade the worm-eaters to supply abundant recruits the following spring.

    Leaf-cursers and other birders are confined to the ground; removed from the treetop battlefield.  Most have stiff necks and eyes tearing from pollen.  Caterpillars conspicuous enough for us to spot were devoured long ago.

    The warblers are on their way.  The trees are ready to burst.  The caterpillars will soon hatch.  Let the leaf-cursers grouse and enjoy the show; but don’t lose sight of the big picture -- the battle above.   The battle’s waged for millennia.

The advantage continuously shifts from plants, to insects to insect-eaters.  The only constant is the preservation of the balance.  Nothing is expendable; nothing except the observer.

This article originally appeared in the May 2008 issue of “The Peregrine”, the journal of the Three Rivers Birding Club.