The Importance of Being Eaten

Early one morning, when the sun had not yet crested a ridge in the east, my friend Tom Meyer was driving into the park where I worked when he spied movement in a field beside the road. He stopped the car and watched. A gray squirrel was picking its way across the clearing, stopping here, pausing there, perhaps hunting for acorns or hickory nuts it had stashed. Sciatica had driven Tom out of bed. Watching the squirrel go about its morning errands helped distract him from the pain.

Just then a gray-brown object streaked into the periphery of Tom's view. It was on a collision course with the squirrel. Tom saw the great horned owl but the squirrel didn't. At the last instant the bird's landing gear went up, talons sprang open, and wings and tail flared. Smack! The squirrel's breakfast became the owl's late-night snack.

Although he spends a great deal of time studying and appreciating birds of prey, Tom was taken aback. Violence had shattered his dawn reverie. Many of us have experienced similar feelings when watching birds at a feeder. One moment all is calm and bright. The next, a Cooper's or sharp-shinned hawk darts in, talons stab, and blood spatters the earth.

We have every right to wonder: Is predation necessary to maintain healthy ecosystems?

As recently as the late 19th and early 20th centuries, before wildlife science had grown beyond the toddler stage, naturalists and just about everyone else tended to divide animals into categories. The good were hailed for their supposedly gentle natures and termed "useful." They sang prettily or ate insects of the sort that plague farmers, or both. Robins, tanagers, and warblers were "good" birds. On the bad side stood full- and part-time predators such as hawks, owls, and crows.

"The toll of life taken by the strong we accept as part of the scheme of things," wrote R.I. Brasher in Birds of America (1936), edited by ornithologist T. Gilbert Pearson. "But wanton destruction revolts us and a duck hawk [peregrine falcon] is sometimes so carried away by lust of slaughter that it will strike bird after bird from a flock of sandpipers and leave the victims where they fall."

These 19th- and early 20th-century writings are full of such disgust for the modus operandi of predatory birds. Susan Fenimore Cooper's Rural Hours (1850) gives us a typical example. The daughter of James Fenimore Cooper loved wild creatures in a general way, but the sight of a raptor catching robins provoked her ire and condemnation. She observes a "tyrannical hawk" raiding a robin's nest, labels it a "monster," and tells readers that it kills with "utmost coolness." Cooper continues: "The arrogance and impudence of the hawk were intolerable to behold. He picked out another nestling from the young brood, and again sailed away.… Alas for the bereaved mother birds, and their mates!

"Their sharp cries of grief were heard throughout the day; the agitation among the feathered tribe in our neighborhood continued more or less until evening."

Henry Thoreau stands tall among the men and women of his time in accepting predators and predation as natural elements of the scheme of things. "I rejoice that there are owls," he wrote in Walden (1854), a statement that would raise few eyebrows in 2000. John Burroughs, America's most celebrated writer on birds in the late 19th and early 20th centuries, began his career with the antipredator prejudices typical of his era. In his early years the farmboy-turned-author shot hawks and owls on sight, just as his father and Catskill Mountain neighbors had. Later, Burroughs metamorphosed into a highly public defender, suggesting in his best-selling nature books that raptors deserved respect and appreciation.

Thanks to Thoreau, Darwin, Burroughs, and more than a century of field and laboratory work by wildlife scientists, we know today that predators and predation are vital not only to healthy ecosystems but also to the well-being of their victims.

Imagine a world without predators. (To keep the focus sharp, let's confine the discussion to birds.) Without owls, hawks, and carnivorous mammals and reptiles to hunt them, birds as we know them today would not exist. Flight almost certainly evolved, at least in part, in response to predation. In theory, reptilelike ancestors of birds could flap into trees and escape earthbound carnivorous dinosaurs, allowing them to survive and more often raise young than did many terrestrial reptiles, and their genes proliferated.

In short, to fly early birds needed motivation and predators gave it to them, at least partially. Flight may have evolved for other reasons, too--to provide access to food above the ground (land plants were evolving ever-greater heights, making the world increasingly three-dimensional), to allow swift movement between far-flung feeding and roosting grounds, and perhaps to supply more certain access to sunshine.

The value of flight in escaping predators is illustrated in reverse on isolated land masses such as New Zealand. Here birds flew in to colonize a Colorado-sized country that terrestrial mammals never reached. Why no mammals? New Zealand is surrounded by a moat called the Pacific Ocean. In the absence of predatory mammals to hunt and devour them, a great many birds lost, or nearly lost, their abilities to fly.

By the time Polynesian explorers arrived in New Zealand nearly 1,000 years ago, the country bustled with flightless species such as moas (about a dozen species of immense birds closely related to the ostrich), several varieties of kiwis, and a huge, flightless parrot known as the kakapo.

New Zealand's Stephens Island wren may have been flightless, too, but we'll never know for sure. This brown, yellow-bellied species, first reported in 1894, inhabited only a single 370-acre island in the Cook Strait. Hardly had the world learned of its existence than the bird, which the lighthouse keeper said "[does] not fly at all," vanished forever. Housecats had been set loose on Stephens Island. The wren had never shared its haunts with such killers, and as the cats multiplied, the bird succumbed. We know what the Stephens Island wren looked like because 15 specimens reside in the British Museum. Every one was carried home by the lighthouse keeper's cat.

Predators give birds impetus to fly. They also exert a powerful influence on the way birds look. The late William Holland Drury explains the mechanism in his book Change and Change (1998). "Predation is a potent selective force," Drury writes, "because predators select from among populations of prey with variable characteristics." In other words, birds of a particular species vary in appearance and other characteristics, and predators enjoy greater success in catching certain individuals rather than others.

The results? Protective coloration to avoid detection--consider the magnificent camouflage of the woodcock. Increased power at liftoff--witness a great horned owl flapping off with a squirrel. Greater maneuverability in the air--think hummingbird. Expertly concealed nests--case in point, the thatch-roofed cup of the ovenbird. Penthouse apartments--crane your neck up at a rookery of great blue herons.

Think of what birds would be like if they had no predators to keep them hopping. Swallows might be flightless. The ruffed grouse could be colored a shocking pink. Bluebirds would surely lack earth-toned breasts and sky blue backs, yellow polka dots might cover the song sparrow, and owls, rather than looking dignified in tweed and houndstooth, might flap around in garish polychrome. Admittedly, birds would probably be more colorful in a world without predators, but they might be sluggish and clumsy.

In natural systems, predators and prey fluctuate in a sort of dynamic balance. If prey numbers drop, predators diminish accordingly. As prey burgeons, predator numbers rise. There are always a relatively small number of predators relative to the ranks of prey. When predators launch attacks, they most often come up empty handed. According to John Terres, a European researcher found that of 688 attacks by hawks in a study area, more than 92 percent ended in failure.

Terres also points to the importance of being eaten in keeping prey populations from increasing wildly. A pair of robins, he says, by producing two broods containing a total of eight offspring each year, could, if their fledglings and fledglings' fledglings grew up and commenced reproducing at the same rate, yield the staggering total of 19 million robins at the end of a decade. Predators are not the only factor limiting such growth, of course.

The combined result of all checks on population size is not balanced, most turn-of-the-millennium biologists will tell you. It is more like a pendulum, swinging back and forth between extremes but not damaging the clock.

Wildlife biologists debate the degree of influence predators have on the numbers of the creatures they feed upon, but there can be no doubt that the effect is significant. Too few predators and prey numbers often rise dramatically. Too many and prey begins to disappear. Ultimately, if predator numbers grow too large, their populations crash along with those of their victims.

During the mid-20th century, black-backed and herring gulls along the Gulf of Maine, fattening on table scraps from landfills, swarmed in ever-increasing numbers. Gulls snatched tern chicks from nests, and as gull numbers soared, once-thriving tern colonies on islands began to disappear. The solution? Ornithologists took drastic measures and poisoned some of the gulls. Tern populations began to rise. The crisis averted, scientists advocated a more gentle, long-term solution--capping landfills and letting gull numbers decline to a point where terns and gulls could coexist as they had in the past.

Which brings us to the issue of predator-prey relations in our own backyards. In recent decades, predators such as red-tailed hawks, sharp-shinned hawks, and coyotes have moved into human neighborhoods in force. The shooting of raptors, so common in the 19th century, is rare and illegal now. Coyotes flourish far beyond the bounds of their range in 1900, nest predators such as the blue jay are a common sight at bird feeders, and bird-eating sharp-shinned hawks make frequent appearances in suburbs.

Is the rise in predators a good thing? Naturalists are inclined to think that it is, at least up to a point. Many of us have seen the result of an environment robbed of its large predators: White-tailed deer multiply themselves into plague proportions, gobbling up ornamental plantings and devastating shrubs and ground plants in forests. Yet there may also be such a thing as too much predation. Just as inflated gull numbers resulted in the decimation of tern colonies, so can certain backyard predators proliferate to a point where they threaten the survival of prey.

To illustrate, let's shift from the theoretical to the practical and ponder the fact that unrestricted bird feeding can lead to a sharp rise in your neighborhood's blue jay and raccoon numbers. Who cares? You will, once you consider the hidden implications.

Blue jays and raccoons are nest predators. Most songbirds raise their young on or within a few feet of the ground. The number of blue jays and raccoons that occur naturally in most areas probably pose little threat to songbird numbers. After all, jays and raccoons are North American natives. The birds whose nests they raid have had thousands of years to develop defenses (camouflage, for example) against them. But inflate populations of blue jays and raccoons by supplying them with an unlimited food source, and ground-nesting ovenbirds and worm-eating warblers may vanish from nearby woods.

The moral? Feed blue jays sparingly, and limit the amount of seed spillage available to raccoons.

Welcoming accipiters such as sharp-shinned and Cooper's hawks to your yard makes sense, but at the same time don't make life too easy for them. A bird-feeding station that lacks cover in the form of shrubs or evergreen trees may become a fast-food restaurant for raptors. Hawks are hungry in winter when most of us put out seed. Like people, they'll zero in on ready sources of fat and protein.

In the wild, songbirds rarely gather to forage at a single point. Food is dispersed and shelter typically lies close at hand. Predators have to work hard to make kills. By contrast, birds coming from far and wide to visit a single bird feeder or feeding station are sitting ducks--unless you supply them with hiding places. There are two ways to do so. Locate your feeders near existing trees, shrubs, and tangles, or get out a shovel and some nursery stock and do a bit of planting.

All is well when prey and predators exist in numbers that allow each to thrive--today, tomorrow, and over the long haul. If you still find the notion of raptors and raccoons attacking small birds troubling, consider this. Prey need predators, just as predators cannot exist without prey. In a world without hunting and killing, ancient forbears would never have evolved into the handsome, high-flying songbirds you know and love today.