HARVARD GAZETTE ARCHIVES

Alberto Pallerino, a postdoctoral fellow in psychology at Harvard University, has foun that pigeons with certain kinds of markings are better at evading attacks by predators. Here Pallerino exercises Biko, his pet African crowned eagle. (Staff photos Peter DiCampo/Harvard News Office)

By carefully cutting, stripping, and gluing, they switched rump feathers between two types of pigeons. Those that got white feathers on their rumps fared much better than those that received blue or gray feathers.

"We were testing a theory about survival," explains Palleroni, a postdoctoral fellow in psychology at Harvard University. "The goal was to determine if certain kinds of markings can protect animals like pigeons from attacks by predators."

As a former student at the University of California, Davis, Palleroni found himself at the right place at the right time to do this. "Our lab was under a pigeon commuter line between the campus and their feeding area," he says.

Over seven years of sky watching, he and others recorded 1,794 attacks by adult and young peregrine falcons on the commuting pigeons. On these high-speed diving assaults, falcons reach speeds of 250 miles per hour, or more.

Biko doesn't live on a diet of pigeons.

In the interest of science (or fun), a friend of Palleroni's skydived with trained peregrines to check their speed. When diving from high altitudes, the birds adjust their speed by holding their wings tightly or loosely against their sides. With wings tightly compressed, a falcon passes a falling skydiver like he was standing still.

For each attack, Palleroni recorded the type of plumage of the pigeon and whether or not the bird was captured. These targets boasted six different types of plumage, identified by markings with nicknames names like "blue bar," "blue checker," "red," and "splash." The researchers were particularly interested in "white backs," blue-gray birds with a solid white patch on their lower back at the base of their tails.

Juvenile falcons captured a pigeon on 19 percent of their dives. The more adept adults scored on 40 percent of their swoops. The most unusual statistic, however, was the fact that white backs accounted for only 2 percent of the captures of both young and adult falcons.

Reversing targets

That result fit well with the idea that certain markings can save an animal's life. White-tailed deer, white-tailed rabbits, and shorebirds with contrasting dark and light surfaces all seem to confuse their attackers this way.

To be sure, Palleroni and some helpers took to pigeon-napping. They nabbed 756 white-rump and blue-bar pigeons and changed their feathers. Simply painting the birds would not trick the falcons because they can see in the ultraviolet range. So the custom feathering required lots of careful cutting, stripping, and gluing.

"We got quite fast at the task," Palleroni recalls, "getting the change down to less than 10 minutes a bird."

After the feather switch, capture rates reversed. Former white backs with faux blue feathers were captured at the same rate as natural blue bars.

With new white feathers, former blues got caught a scant 2 percent of the time.

"This result shows that the primary factor underlying survival of wild pigeons from attacks by peregrine falcons is the presence of the white rump patch," the researchers report in the April 24 issue of Nature. "In the evolutionary arms races, rump tricks win survival," is the way Palleroni puts it.

But how? To keep from being eaten, all pigeons take the same evasive action. They drop one wing and roll out of the falcon's path. From the point of view of a falcon, it is flying at high speed toward the back of the pigeon, probably fixed on its rump. "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," Pallerino notes.

"High speed plays an important role," he continues. At 250 miles an hour, the falcon outflies its vision: It moves faster than its brain processes what it sees. Palleroni estimates that it takes about 1/50th of a second for the pigeon's rollout to register in its birdbrain. "For a short time, the falcon is kind of flying blind," he comments.

"When pursued by predators, shorebirds, such as sandpipers, and schools of fish alternate displays of dark and light surfaces to confuse predators," Palleroni adds. Deer and rabbits with white tails may use a similar strategy.

Becoming targets

That explanation raises the question of why white backs don't dominate the pigeon world if other feather patterns are being falconed out of the sky at such high rates. Female choice is one answer. White-back females don't limit their selection of a mate to like-plumaged males. They couple more frequently with pigeons wearing other color patterns.

Palleroni and his colleagues did find a significant increase in white backs relative to blue bars over the course of their study. However, that occurred in high, open skies dominated by peregrines, the fastest of falcons. White backs don't fare as well with lower, slower falcons and hawks. Palleroni's own scholarly aerie overlooks well-known Harvard Yard where red-tailed hawks - hunters of pigeons and squirrels - soar and swoop lower and slower than peregrines. Here, pigeon-nappping is not necessary to show that what's an advantage in one place can be a disadvantage in another. All it takes is some idle window gazing to see that a white rump can become a target instead of a life-saving distraction.