Preying peregrine falcons plunge through the sky at more than 250 miles per hour, leveling out at the last minute to knock speeding pigeons out of the air in an explosion of feathers. But the victim's feathers are rarely white. A new study shows that pigeons with white rumps have a strong advantage over their grey-butted brethren in avoiding death from above--perhaps because a bright derriere distracts the pursuing bird.
As anyone who has watched a flock of pigeons in the park knows, the birds come in several colors and patterns. This multitude of markings is maintained because pigeons tend to choose mates with plumage unlike their own. But after hearing a talk about the role of markings in predator evasion, Alberto Palleroni, then a Ph.D. student in ecology at the University of California, Davis, began taking notes from his lab window on which pigeons were attacked by falcons as they "commuted" between the university and nearby feed lots.
Over seven years, Palleroni, now at Harvard, and colleagues recorded nearly 1500 adult falcon attacks on pigeon flocks near Davis, 40% of which resulted in kills. Although white-rumped pigeons make up 20-25% of the pigeon population, they represented only about 10% of falcon attacks and less than 2% of kills, the researchers report 21 April in Nature. The team then captured more than 750 white and gray-rumped pigeons and swapped their hind feathers using rubber cement. When the researchers released the pigeons, Palleroni says, "the survivorship completely flipped."
The white rump may act as a distraction to a falcon, say the researchers, providing a focal point for the falcon's attention and preventing it from noticing when the pigeon drops one of its dark-colored wings and rolls out of the peregrine's fight path. As once-endangered peregrine falcon populations continue to increase [ScienceNOW 6 May, 1998], this selection pressure may play a growing role on the pigeon population. It may have already begun: Over the course of Palleroni's experiment, the number of falcon attacks increased, as did the proportion of white-rumped pigeons in the population relative to their gray-rumped peers.
The findings "suggest the pigeon has a really good reason for having a white rear end," says Steven Lima, an ecologist who studies predator-prey interactions at Indiana State University in Terre Haute. Lima hopes the researchers will continue to follow the birds over successive generations to see if the pigeon population continues to change under the falcon's selection pressure. That, he says, would be "evolution in action."