which have invaded nearly every terrestrial environment on earth, are unable to evade the birds that probe the soil, turn over the leaf litter, search the bark, dig into the trunks of trees, scrutinize every twig and living leaf. The water is no safe refuge, nor is the air, nor the dark of night. There is a bird of some sort to hunt nearly every insect. Warblers and vireos methodically work the leaves while swallows, swifts and other hunters of flying-insect prey spend most of their waking hours on the wing, ranging hundreds of miles daily in their aerial forays.
This story began about 155 million years ago with the first known bird, the famous Archaeopteryx, represented by beautiful, complete fossils from a limestone quarry in Bavaria. Archaeopteryx combined avian and reptilian characteristics, which shows—as do more recently discovered feather-bearing dinosaur fossils from China—that the birds are direct descendants of the dinosaurs. During the next 120 million years many species of birds evolved, but relatively few of them were insect eaters. During the Miocene epoch, beginning about 30 million years ago, the rapid evolution of the flowering plants and the hundreds of thousands of insects that exploit them resulted, as Frank Gill noted, in an explosive evolutionary radiation of insectivorous birds, mainly songbirds (order Passeriformes), which today constitute close to six thousand of the almost ten thousand known species of birds.
Most birds include insects in their diet. With few exceptions, most notably pigeons and doves, even the most dedicated vegetarian birds, fruit and seed eaters such as finches, buntings, grosbeaks, and cardinals, feed their nestlings a high-protein diet of animal matter, mainly insects. The behavior of a cardinal observed by Josselyn van Tyne is illustrative: “At noon on May 24 the adult male, on his way back to the nest territory, stopped at my feeding shelf with his beak full of small green worms [caterpillars] such as I had often fed to the young. He immediately put the worms down on the shelf and began cracking and eating sunflower seeds…. He then picked up the worms, flew across the street, and (presumably) fed the young.”
Insect eaters avoid competition by sharing the environment, specializing in where and how they hunt. Ground feeders such as towhees and fox sparrows search the litter on the forest floor for both insects and seeds; warblers, chickadees, and other leaf gleaners search foliage for caterpillars and other insects; nuthatches and brown creepers are among the bark gleaners; wood and bark probers, such as woodpeckers, bore into trees to find grubs and other burrowing insects; flycatchers and, on occasion, many other birds are air salliers that dash from a perch to snatch insects from the air; and finally, gleaners of aerial plankton, among them swallows, swifts, and nightjars, more or less constantly swoop through the air to scoop up flying insects.
As Peterson elaborated, avian insectivores have become specialized to exploit insects as food in many different ways. Take, for example, the many birds that snatch flying insects on the wing. Most of the thirty or more flycatchers (family Tyrannidae) that nest in the United States and Canada wait quietly on a perch, often a bare branch with a clear view of the surrounding airspace, from which they dash out to snap up passing insects. They usually return to the same or a nearby perch to wait for another insect to fly by. One of the most familiar of the tyrant flycatchers is the eastern kingbird, often seen perched on fence wires along country roads. This gray and white bird, truly a pugnacious tyrant, attacks and chases away any birds that come too close to its territory, even crows, hawks, and vultures. From Nebraska south to Texas, the exquisitely beautiful pink and pearly gray scissor-tailed flycatcher, its forked tail twice as long as its body, is a “wire bird” that likes to perch on telephone lines along the roadside. The little brown tail-pumping eastern phoebe, which often builds its mud-based nest under a bridge or on a rafter in an outbuilding, is one of the first birds to return to the northeastern United States, as early as March. “Now and then,” Edward Forbush and John May wrote, “one of these early birds may be seen darting out from its perch in a March snowstorm, apparently catching insects.”
Tyrant flycatchers will eat almost anything that flies, as well as small caterpillars and spiders that “balloon” through the air on long strands of silk. Several species even eat wasps and bees, because they can avoid being stung or can recognize and catch only stingless males. Some beekeepers believe that the brown-crested flycatcher, which preys on bees in apiaries, is a pest that eats worker bees. A discerning Arizona beekeeper, quoted by Herbert Brandt in Arizona and Its Bird Life, at first agreed “but after examining the stomach contents of a large number of [brown-crested flycatchers] during a period of more than 20 years, and not in a single instance finding the remains of a worker bee, nor finding a bee sting in the mouth or throat of one of these birds, became convinced that [they] did not prey on worker bees, but only on drones [the stingless males].”
Many birds are opportunists that will flycatch when a flying insect comes temptingly close. I have seen warblers of several species put their search for caterpillars on hold as they darted out from a leafy branch to catch a passing fly. Even a nuthatch crawling on a tree trunk will stop searching for insects on the bark to pursue a flying insect. Cedar waxwings feed mostly on small fruits such as chokecherries and blackberries. But “in late summer and early fall,” Forbush and May noted, “the cedar waxwing turns to flycatching, and taking its post on some tall tree, usually near a pond or river, launches out over water or meadow in pursuit of flying insects.” Birds caught at such times have been found crammed to the very beak with insects.
The birds commonly called goatsuckers (order Caprimulgiformes, from the Latin roots for goat and milk) are seldom-seen night fliers that eat almost nothing but insects. (Their odd common name stems from the European myth that they suck milk from goats.) At night, the loud and clearly enunciated calls of most of the North American goatsuckers not only make their presence known, but tell us their common names, which are verbal renditions of their calls: whip-poor-will, poor-will, and chuck-will's-widow. The common nighthawk is an aberrant goatsucker that flies both at night and during the day and, unlike the others, is likely to live in a city, laying its well-camouflaged eggs not in a nest but on the surface of a flat, gravel-covered roof.
Goatsuckers have wide, gaping mouths with which they scoop insects from the air. Except on the nighthawk, stiff bristles surrounding the mouth expand the “scoop.” Goatsuckers eat night-flying insects of almost any kind; among others, mosquitoes, June beetles, flying ants, and moths, including even large moths with 4- to 5-inch wingspans, such as cecropia, luna, and polyphemus. Larger than the other goatsuckers at a length of almost a foot, the chuck-will's-widow occasionally eats small birds. The goatsuckers are the night shift of the birds that harvest the many night-flying insects and ballooning spiders that crowd the air. As we will soon see, they are joined by bats, which fly much higher than most of the goatsuckers.
The swifts, the swallows, and the nighthawks—the last working days only part-time—are the day shift of the birds that subsist on aerial plankton. There is little competition between them because nighthawks are seldom numerous and the swifts and the swallows, which are usually very numerous, have found a way to share the airspace. Swallows tend to hunt a relatively short distance above the ground, usually at a height of only a few yards. Swifts, on the other hand, fly much higher. Chimney swifts, the only members of their family in eastern North America, live in towns and cities and, almost constantly on the wing from sunrise to sunset, fly well above the tallest church steeples. Before the advent of Europeans in the New World, chimney swifts glued their twig nests to the inner walls of hollow trees. Today practically all of them prefer to nest in chimneys.
Several of the eight North American swallows have also become associated with humans. Most of us view the colorful, fork-tailed barn swallow, the most familiar of these birds, with affection. They usually nest inside a barn or other outbuilding, sticking their feather-lined mud nests to a wall or placing them on a rafter or some other support. An opportunistic hunter, a barn swallow takes advantage of every occurrence that makes insects easy pickings. Forbush and May wrote that “it follows the cattle afield or swoops about the house dog as he rushes through the tall grass, and gathers up the flying insects disturbed by his clumsy progress. When the mowing machine takes the field, there is a continual rush of flashing wings over the rattling cutter-bar just where the grass is tumbling to its fall.
