The feral or rock pigeon doesn’t have a lot of fans. True, these birds add a touch of nature to the urban jungle; when I worked in San Francisco, I looked forward to the occasional pigeon visit to the window ledge outside my cubicle. But I still harbored a grudge against the one that guano-bombed me in the Piazza di San Marco in Venice years ago, and tended to think of them as feathered vermin.
Alberto Palleroni, a former UC Davis doctoral candidate now a post-doc at Harvard, sees it differently. “The feral pigeon is an amazing balance of adaptations and success,” he told a New York Times reporter earlier this year. “I’m always one to defend them.”
Pigeons have intrigued scientists for a number of reasons. That homing instinct, for example, which has been exploited since antiquity. Roman magistrates took pigeons along to the theater in case they were delayed and had to send word home. Pigeons carried the results of the ancient Olympics, relayed Genghis Khan’s battle orders, and linked besieged Paris with the outside world in the Franco-Prussian War. The British Air Force employed 20,000 homing pigeons in World War I. As of a couple of years ago, at least, a Police Pigeon Service still functioned in the Indian state of Orissa.
The birds seem to use a mix of navigational tools. Tiny particles of magnetite in their beaks help them orient by the earth’s magnetic field, with assists from their sense of smell and possibly their perception of low-frequency sound.
Psychologists, from B. F. Skinner on, have also explored the mental world of the pigeon. At Cardiff University in Wales, John Pearce discovered that they can discriminate between pictures of trees and pictures of non-trees, then went on to establish that they can tell Picassos from Monets—pattern recognition on a pretty sophisticated level. What interested Palleroni, though, is the pigeon’s own plumage patterns, and how one particular variant helps protect the birds from that avian superpredator, the peregrine falcon.
In the centuries after the first wild rock pigeons started hanging around Middle Eastern villages for spilled grain and other treats, their descendants have been tweaked into a dizzying variety of plumages, shapes, and behaviors: aerobatic tumblers, bizarrely feathered Jacobins, pouters, shakers. Charles Darwin used pigeon breeding as a model for the shaping force of natural selection. With typical Darwinian thoroughness, he joined two London pigeon clubs and tried his hand at breeding. You could argue that pigeons had more to do with the origin of The Origin of Species than Darwin’s famous finches. Today’s street pigeons are reasonably uniform in shape, but diverse in plumage pattern. Most have uniformly dark backs, but a minority—20 percent in Palleroni’s study—have a white rump patch.
Palleroni, a serious falconer who works with an African crowned eagle named Biko, decided to study peregrine predation on Davis’s rock pigeon population. His lab was strategically located under the birds’ flight path between the campus and a favored feeding location. Over seven years, Palleroni and his associates logged 1794 peregrine predation attempts on the commuting pigeons. In a typical attack, the falcon barrels down on its target at a speed of up to 250 miles per hour, then levels out and hits the pigeon from behind, killing or stunning it with a blow from its clenched talons.
Adult peregrines were found to have a higher success rate than juveniles (40 percent versus 19 percent). One unanticipated finding was that white-backed pigeons made up only 2 per cent of the total kill for both falcon age classes—much less than a representative share of the pigeon population. To eliminate the possibility that white-backs were simply better at dodging, Palleroni did an ingenious cut-and-paste experiment, trapping 756 pigeons of the white-back and the more common blue-bar forms and changing the color of their rump feathers. He couldn’t just paint the birds, since peregrines can see into the ultraviolet range and would have detected the fraud; he had to excise the pigeon’s rump plumage and glue in feathers of the opposite color. Once up to speed, it took his research team only 10 minutes to customize a pigeon. When released in falcon territory, former white-backs now sporting blue rumps suffered heavy casualties, but former blue-bars with white rumps were caught just 2 per cent of the time.
What happens, according to Palleroni, is that the diving falcon fixates on the conspicuous white rump patch. While the raptor closes in, the pigeon executes its standard evasive maneuver, dipping one wing, rolling, and veering off like a jet fighter breaking formation. At 250 mph, the falcon is moving faster than its brain can process what it sees. It takes 1/ 50 of a second for the pigeon’s dip and roll to register. That’s apparently all the time the pigeon needs. “In effect, it’s a kind of card trick,” Palleroni says. “The patch may disguise the start of the evasive roll, confusing the attacker with the sudden contrast between conspicuous white and dull gray-blue body.”
Trying to baffle a predator by breaking a visual pattern is not uncommon among prey species. The white flags of the cottontail rabbit and the white-tailed deer serve a similar purpose. If you’ve ever watched a falcon going after a flock of shorebirds, you’ve seen the dazzling reversal of dark backs and white bellies as the birds twist and turn in flight. Schools of baitfish pursued by bigger fish do the same thing.
So it makes sense that the white-back pattern would confer enough of a survival advantage for the responsible genes to be preserved. But it seems the trick only works for peregrines, old familiar enemies of the rock pigeon; the two evolved side by side in Eurasia before pigeons spread worldwide. According to Palleroni, the red-tailed hawks that patrol Harvard Yard have no trouble at all catching white-backs.