up flying insects—even the rush and roar of that modern juggernaut, the motor-car, has no terrors for it.”
Other birds take advantage of similar opportunities. In spring I sometimes see flocks of ring-billed gulls in recently plowed central Illinois fields, harvesting soil-dwelling insects turned up by the plow, probably including fat cutworm caterpillars that would have become night-flying moths; C-shaped white grubs, the larval stage of June beetles; larval click beetles, the skinny, brown wireworms; perhaps the overwintering pupae of a corn earworm moth. In Africa, cattle egrets snatch insects flushed up by large grazing animals. We also see these birds associated with grazing cattle in southern Ontario and most of the continental United States. They were first seen on this side of the Atlantic in northern South America about one hundred years ago, a flock probably aided by the trade winds having crossed the ocean. In tropical America, ant birds (family Formicariidae) follow swarms of army ants, feeding on insects the ants flush up as they advance over the forest floor.
Birds that eat insects associated with trees, as we have seen, can be grouped into several quite different feeding guilds. These specializations are driven by competition between species, which forces birds to share the available insects. In 1934, G. F. Gause pointed out that “as a result of competition, two similar species scarcely ever occupy similar niches, but displace each other in such a manner that each takes possession of certain peculiar kinds of foods and modes of life in which it has an advantage over its competition.” This is the competitive exclusion principle.
“Species that coexist in seemingly homogeneous habitats, such as grasslands or spruce forests,” Frank Gill wrote, “may segregate their niches even more finely.” Five insectivorous wood warblers, colorful species that migrate back and forth from the New World tropics to where they nest in the spruce forests of the north, manage to coexist on the same spruce trees by feeding in different ways on particular parts of the trees, as Robert MacArthur discovered. Gill's concise summary of MacArthur's observations explains how the warblers avoid competing with one another: “The yellow-rumped warbler fed mostly in the understory below 3 meters [almost 10 feet], the black-throated green warbler in the middle story, and the blackburnian warbler at the tops of the same spruce trees. Sharing the middle part of the trees with the black-throated green warbler, which explored the foliage for food, was the Cape May warbler, which fed on insects attracted to sap on the tree trunk. Sharing the treetops with the blackburnian warbler, which fed on the outer twigs and sallied out after aerial insects, was the baybreasted warbler, which searched for insects close to the trunk.”
In addition to almost all of the fifty species of warblers that can be seen in the United States and Canada, other birds also make a living by searching foliage for insects. A few among the many are the vireos, orioles, tanagers, kinglets, titmice, and chickadees. 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
