Summer World_ A Season of Bounty - Bernd Heinrich [38]
ONCE, WHILE STILL SEARCHING FOR CATERPILLARS, I SAW something that made my eyes pop. It was in midsummer, and I found, under tall forest trees, partially eaten green leaves on the ground. When I picked them up and examined them, it was obvious that they had not been shed by the tree in the usual way (at the junction between twig and leaf stem). They had been cut through the stem (petiole); caterpillars had fed on the leaves and had then snipped off the remainder. They were discarding the remnants of their meals, and they had used precious time and energy to chew through the very tough woody leaf stems. Since I had myself been using leaf damage as a cue in my caterpillar hunting, it seemed to me that caterpillars leaving feeding damage on leaves would effectively be leaving a “track” that caterpillar-hunting birds might use to find them. Obliterating those tracks would be a neat trick for “invisible” caterpillars, to keep predators at a distance. Like the blues’ caterpillars and their ants, the story of leaf-eating caterpillars and birds also involves an evolutionary arms race, and it is waged all day every day all summer long.
Birds have specialized behavior to capture insects, and insects have specialized behavior to try to avoid capture. Throughout evolutionary history, the field of battle in this arms race keeps shifting as each participant keeps up with the other. Those that don’t keep up will cease to exist. At no time is the race more intense than after the birds’ young hatch, when the parents’ real work of caterpillar hunting begins.
Most small northern forest birds attempt to raise four to six young in any one clutch. It requires an enormous daily investment in foraging to feed that many babies and bring them to adult body weight in about a week. To predators, baby birds are helpless gobs of tender meat whose noisy competition among themselves for their parents’ attention—in order to be fed—is nothing more than a convenient advertisement of where to find them. Thus the fledgling stage is the most dangerous time during a bird’s life, and there is a huge advantage in becoming capable of flight and getting out of the nest as soon as possible. To maintain the offspring’s phenomenal, sustained growth spurt, the adults must feed their youngsters every few minutes, and the food must be easily digestible protein. For most forest birds, that means caterpillars.
Much of what applies to baby birds also applies to caterpillars, of course, except that they necessarily feed on foliage, a decidedly low-protein diet. Caterpillars have few hard parts—no skeleton and usually no “fur.” They are easy to digest, and many of them require no preparation before eating: they can be swallowed as is. Like baby birds, they also need to grow fast, but because most (though not all) of them have a vegetarian diet, they don’t reach their full weight nearly as fast as baby birds. For many caterpillars survival requires a delicate balancing act—hiding versus feeding. This is a hard compromise, because the tree’s leaves are necessarily exposed to sunlight, where it is difficult to hide.
Birds, wasps, and flies have been preying on or parasitizing caterpillars (or both) for probably at least 100 million years. Year in and year out, the great majority of hatchlings of any one clutch of moth or butterfly eggs, consisting of perhaps two hundred, will be eaten. Such relentless pruning has necessarily left a deep imprint on the caterpillars.
Caterpillars are startlingly, dazzlingly diverse in their shapes, colors, and behavior. Some are protected by potent poisons. Others have greatly reduced their palatability by developing woolly hairs or sharp bristles. All the caterpillars that are conspicuous to us and to birds are relatively immune to wasp and bird predation; those that are hard to find tend to be the most prized as food by birds. Not surprisingly, therefore, the majority of edible caterpillars use a variety of strategies to keep themselves well hidden, if not to make themselves nearly invisible,