To Catch A Thief
When biologists study food theft among endangered roseate terns, they find that crime most definitely pays
Patrolling the June skies above Long Island Sound’s tiny FalknerIsland, one roseate tern spies another flying 15 feet below with a fish in its beak. The first bird dives, grabs the fish and repairs to a nearby pebbly beach to feed its chicks.
“That’s Supertern,” says biologist Dave Shealer of the fish thief. “Good catch!” says Shealer’s colleague, Jeff Spendelow, who, peering through a spotting scope, reads an identifying number on a band around the bird’s leg. The men—Shealer, a 37-year-old behavioral ecologist based at Loras College in Dubuque, Iowa, and 53-year-old Spendelow, a U.S. Geological Survey research biologist at Maryland’s Patuxent Wildlife Research Center—are shoulder to shoulder in a small bird blind on the island’s north end.
Thanks to these two researchers, Supertern and several other FalknerIsland roseate terns are among science’s most celebrated practitioners of kleptoparasitism, or food theft, from the Greek word kleptes, meaning thief. The practice of stealing food to feed themselves, their young or a mate— common to other bird species such as frigate birds, parasitic jaegers, skuas and several types of gulls—was first noticed in roseate terns in 1973. But Shealer and Spendelow’s recently published research has shed new light on this unorthodox behavior. They have found that, contrary to conventional wisdom, crime does pay—at least among roseate terns.
Spendelow and Shealer’s “laboratory,” about three miles off the Connecticut coast, is a slip of an island consisting of four and a half acres of rocky beach, poison ivy, six-foot-high black mustard plants and staghorn sumac trees. The north end of the banana-shaped island rises 60 feet from the sea like the prow of a ship; the island’s midsection is marked by a 200-year-old working lighthouse. But birds dominate the landscape: nearly 3,000 pairs of common terns and 45 pairs of their smaller, endangered cousins, the roseates, make their nest here. Much appreciated by bird lovers, roseates have a slightly rosy breast that glows “with the faintest blush of some rare seashell,” wrote early 20th-century ornithologist Arthur C. Bent. Their effortlessness in the air prompted John James Audubon to describe them as “the Hummingbirds of the sea.”
Spendelow first began studying roseates on Falkner 25 years ago. In 1987, he devised an ingenious system of banding that did not require recapturing the birds to identify them: unique combinations of colored plastic bands that can be seen with a spotting scope from as far as 75 yards away. Spendelow estimates that he and his colleagues have outfitted more than 50,000 roseates in Connecticut, Massachusetts and New York with these bands. Using this system, Spendelow has been able to make detailed histories of thousands of individual roseates and monitor the ups and downs of their population.
In 1994, Spendelow invited Shealer to come to Falkner. Shealer had just spent four summers researching how roseates in Puerto Rico forage for food and had observed some terns stealing fish from others. He wondered whether this was normal behavior or just opportunism. Shealer felt that Spendelow’s banding system would enable him to distinguish one bird from another and thus answer this question.
During the next four years on Falkner, Shealer would fold his 6-foot-1 frame into the bird blind for a total of 774 hours to watch the terns. “When you get to know them as individuals, you can’t help but anthropomorphize,” he says. “There’s one that abuses her offspring, one that [acts like he’s] God’s gift to terns, another that loves mackerel. There are devoted parents, and a male that courts anything that moves.”
Shealer found that kleptoparasitism is indeed a way of life for some roseates, if only for two males and eight females, or 4 percent of what was then FalknerIsland’s nesting roseate population. The thieves employed a number of scams and tricks. Supertern and a female dubbed the Good Mother swooped on their quarry from above or slashed from below. Other klepto terns—sly as purse snatchers—pre ferred to jump their victims on the ground. Afew crafty females bided their time until a neighbor arrived home with a fish, then blocked the neighbor’s access to its chicks and grabbed the fish. “One brazen female actually solicited sex repeatedly,” Shealer adds. She would flirt with a fish-laden male by being submissive and begging, behavior that is a prelude to mating. “When the foolish male would fall for it and attempt a mounting on her back in preparation for copulation,” says Shealer, “she tilted her head upward, snatched the fish from the distracted suitor and took off without so much as a backward glance.”
There were other surprises. Shealer and Spendelow often watched Supertern bring several fish in rapid succession to his mate. Once she was satiated, he would then fly off to visit other, single females. After copping a fish from some unsuspecting tern, he’d offer the stolen fish—the tern equivalent of a box of chocolates—to female after female, until he won what biologists call “extra pair copulation.” Spendelow says he “wouldn’t be surprised if Supertern fathered half of the chicks” of single females on the island.
Klepto terns also make good parents. Honest terns bring their chicks only two young three-inch-long sand lance fish per hour. But Supertern, for example, can bring his mate’s offspring up to 20 per hour. Consequently, Supertern and the nine other thieving parents have healthier chicks, which are far more likely to survive and have chicks of their own. This, says Shealer, makes the outlaws the big winners in the game of life, where success is measured by the ability of individuals to pass their genes along to the following generation.
But do the birds also pass along their thieving ways? Apparently not. Shealer and Spendelow have yet to see the offspring of a klepto tern resort to stealing. Many questions about kleptoparasitism remain: Why, for example, do terns steal rather than fish for themselves? What are the effects on honest roseates in the population?
Sitting in the blind, watching over the terns, Shealer offers a few speculations. Just as no two humans are alike, he says, neither are two birds. A bird that sees its neighbor fly in with a fish might be tempted to steal it, he says, “but I believe that kleptoparasitism is not just a matter of opportunism.” Nor, he says, is it laziness—a way to avoid the up to an hour-long, 25-mile round-trip flight out to sea to search for sand lances. Perhaps, Shealer speculates, some birds simply discover that they are good at stealing. It does, after all, require exquisite skill, speed and timing. Others might lack the ability and give up after trying once or twice.
Whatever motivates terns to steal, it’s clear that “kleptoparasitism is not a loser’s strategy,” says Shealer.