How Not to Be Eaten. Dr. Gilbert Waldbauer
The spry little black-capped chickadees are agile acrobats that nimbly hop from twig to twig and may hang upside-down as they inspect a leaf for their next meal, which is likely to be a caterpillar. They are particularly interesting because they display what—at least in my view—can only be called intelligence as they search for their prey. Their hunting behavior, Bernd Heinrich and Scott Collins found, is amazingly clever and sophisticated. As we will see in chapter 9, they keep an eye out for partially eaten leaves—those that are tattered or holey—which, they realize, indicate that caterpillars are probably nearby.
Some birds are preoccupied with the trunks and larger branches of trees: bark gleaners, such as the brown creeper and white-breasted nuthatch, and bark and wood probers, such as woodpeckers and, of all things, a very unusual finch on the Galápagos Islands. The brown creeper has an energy-efficient way of searching a tree trunk for insects, spiders, other small creatures, and their eggs tucked away in the crevices of the bark. It begins at the base of the trunk, which it climbs up in a spiral path while conducting its inspection. When it is ready to move on, it spreads its wings and, expending a minimum of energy, glides down to the base of a nearby tree trunk and begins another upward climb as it hunts for food. The white-breasted nuthatch frequently crawls headfirst down the tree trunk. From this perspective it is likely to find food that brown creepers miss. In winter, nuthatches supplement their diet with plant food, such as acorns and sunflower seeds, which they often conceal in bark crevices for future use, a behavior that inspired their common name.
The woodpeckers (twenty or more species in North America), Roger Peterson wrote, “spend most of their lives in a perpendicular stance, clamped against a trunk or a branch, the stiff tail acting as a brace and the deeply curved claws, two forward, two aft on each foot, clutching the rough bark. The straight beak, hard as a chisel, is driven in triphammer blows by powerful muscles in the head and neck.” The beak is used to find wood-boring insects by gouging into solid wood, and to excavate its own deep nesting cavities. Beetle grubs and other insects are extracted from their burrows by a barbed tongue that can extend as much as five times the length of the bill, a tongue so long that it can be stored in the head only by looping around the skull. A physician quoted by Steve Nadis wondered what makes it possible for these birds to use their head “as a battering ram without sustaining headaches, concussions or other brain injuries,” why dead and dying woodpeckers don't litter the countryside. Dissecting woodpecker heads has yielded some answers, among them a tightly fitted skull that keeps the brain from banging around and shock-absorbing muscles that encircle the skull.
The remarkable woodpecker finch is one of a group of fourteen finch species found only on the Galápagos Islands. Discovered by Charles Darwin in 1835, these birds, commonly called Darwin's finches, are very different from one another in feeding behavior and have beaks appropriately adapted to handle what they eat. Among them are species that feed on insects, seeds, leaves, nectar, or the pulp of cactus pads, according to David Lack. It is generally agreed that all of them evolved from a single colonizing flock of one species that somehow crossed 600 miles of the Pacific Ocean from the closest point on the South American mainland to the recently (geologically speaking) volcanically formed and at first lifeless islands. With few other birds to compete with them, they avoided competing with one another by evolving ways of exploiting unoccupied ecological niches.
An ornithologist working on the Galapágos in 1914 was the first to observe woodpecker finches using tools, Lack noted. Although they peck holes into trees to find wood-boring insects, they lack the long extensible barbed tongue with which true woodpeckers extract beetle grubs or other insects from their holes. Instead, as Sabine Tebbich and her colleagues reported, woodpecker finches “use twigs or cactus spines, which they hold in their beaks…to push, stab or lever [insects] out of tree holes and crevices…. Moreover, they modify these tools by shortening them when they are too long and breaking off twiglets that would prevent insertion.”
A few other birds, perhaps two or three dozen species, are known to use tools, but only a handful use them to capture insects. Jeffery Boswall noted three Australian birds—the shrike-tit, the grey shrike thrush, and the orange-winged sittella—that use twigs to probe for insects in crevices. In Tangipahoa Parish in Louisiana, Douglas Morse watched brown-headed nuthatches pry pieces of bark from longleaf pines with flakes of bark to get at hidden insects.
Nathan Emery and Nicola Clayton, in an article in a 2004 issue of Science, wrote that wild “New Caledonian crows…display extraordinary skills in making and using tools to acquire otherwise unobtainable foods.” Tools for extracting insect larvae from holes in trees “are crafted from twigs by trimming and sculpting until a functional hook has been fashioned.” Other tools, “consistently made to a standardized pattern” by cutting pieces from Pandanus leaves, are used “to probe for [insects] under leaf detritus [with] a series of rapid back-and-forth movements that spear the prey onto the sharpened end or the barbs of the leaf.” On foraging expeditions, the crows carry these tools from place to place. One caged New Caledonian crow, Emery and Clayton noted, appeared “to be capable of reasoning by analogy with her previous experience with hooks, by modifying nonfunctional novel material (metal wire) into hook-like shapes to retrieve food.”
In winter, cohesive flocks of black-capped chickadees, tufted titmice, nuthatches, brown creepers, golden-crowned kinglets, and downy woodpeckers wander through the woods foraging for insects. The birds, so to speak, gang up on the insects. Field studies summarized by Kimberly Sullivan “showed that individuals can benefit from membership in a flock by decreasing their risk of predation and increasing their foraging efficiency.” Flock members constantly sound contact, or social, calls—such as the chickadee's chick-a-dee-dee—that announce their presence and help to maintain flock cohesion. They also have calls, such as the chickadee's high-pitched zeee, that warn of an approaching hawk or other predator. Most small birds, according to Susan Smith, freeze or dive for cover in response to the warning calls of their own and other species. Sullivan, as we will see in chapter 9, found that downy woodpeckers spend much more time eating and much less time cocking their heads from side to side to watch for predators when they are with a flock than when they are alone, because the constant contact calls of their companions assure them that others are also keeping an eye out for predators.
Many of the 4,500 species of mammals—from tiny shrews, bats, and mice to huge bears—feed on insects to varying extents. Some, such as the African aardvark and the giant anteater of South America, eat nothing but ants and termites. Many omnivores, among them bears, raccoons, opossums, chipmunks, foxes, squirrels, mice, and skunks, include insects in their diets.
Primates such as lemurs, tarsiers, monkeys, baboons, chimpanzees, and humans are omnivores that, to varying degrees, feed on insects. In the early 1960s, Jane Goodall made the famous discovery that chimpanzees create tools from twigs and use them to “fish” for one of their favorite snacks, termites—the tropical species that build large cementlike mounds. Early in the rainy season, swarms of thousands of male and female termites of the reproductive caste leave the mounds through tunnels dug by workers, who keep the exit holes thinly sealed until conditions are favorable for the reproductives to fly off and found new colonies. When a hungry chimpanzee spots one of these lightly sealed holes, Goodall observed, it removes the seal with its index finger and pokes a tool into the hole. A moment later the chimpanzee withdraws the tool and then eats the termites clinging to it. Children in Africa use the same technique to get a few termites for a snack, but adults on the continent make ingenious traps to catch swarming termites—a much-favored food, delicious when roasted—by the thousands. As I explained in Fireflies, Honey, and Silk, people of almost all non-Western cultures eat insects, usually as a special treat.
Bats, the masters of the night sky and the only mammals capable of true flight, are not blind, but they find their way in the dark by means of echolocation (sonar). In flight they emit sounds too high-pitched for our hearing and sense obstacles and their prey, usually insects, by listening for the echoes that bounce back from them. As the Nobel laureate Niko Tinbergen observed, on what is to us a quiet summer evening, to the bats flying about and the moths that can hear them “the evening is anything but calm. It is a madhouse of constant shrieking. Each bat sends out a series