If homing pigeons wonder why humans are always driving them to faraway spots and leaving them behind, they don't hold it against us. They just keep coming back, providing prize money for pigeon racers and new data for scientists studying the navigational powers of an avian brain. Now those scientists have discovered a new trick in pigeons' homing toolkit: the birds learn best when traveling near a boundary. A homing pigeon that's looking for its loft can use many kinds of clues. It may navigate by the sun, the earth's magnetic field, or the smell of a nearby chocolate factory. It might use visual landmarks, too. "The birds seem to return well to the spires in the center of Oxford," for example, says Uppsala University mathematics researcher Richard Mann. However, Mann says, "What has been missing is a direct link from something we can measure about [any] landscape...to a measurable change in the birds' navigational behavior." So Mann and his colleagues studied the flight paths of 31 homing pigeons, recorded by tiny GPS devices the birds wore on their backs. Each pigeon flew home 20 times from one of four different sites around Oxford. The birds had never been released at these sites before the study began. Using an aerial photograph of the landscape around Oxford, the researchers calculated how simple or complex the bird's-eye view was from one pixel to the next. They did this using "edges," or "changes in intensity in the image," Mann explains. "You get a lot in cities and in forests," while an open field has fewer.
Left: Paths to the pigeons' home loft from each of the release sites, ranging from 5 to 10.6 kilometers away. Right: The same aerial image filtered to show edges, or complexity. Boundaries between dark and light areas were where the birds learned best. Then they calculated how similar each flight was to the flight taken right before it. As a bird repeated the same trip over and over, how quickly did it memorize the route it would stick to? A bird's faithfulness to one route, the scientists saw, depended on the complexity of the landscape it was flying over. Very complex spaces with lots of edges—like the middle of the city, or dense areas of forest—weren't ideal. (Previous studies have found that it's hard to teach pigeons a route over a city, and that the birds avoid flying over certain patches of woodland.) But empty spaces weren't great either. There seemed to be a kind of Goldilocks landscape, not too dense and not too open, over which the pigeons learned best. Mann says you can picture this ideal scenery as "a view that was half open field, half houses or dense trees." In other words, a boundary between an open area and a city or a forest. "I think it is likely that the pigeons are [using] these large-scale boundaries as landmarks," Mann says, when they navigate their way home. Mann has mostly moved on from navigation research and is now studying collective behavior in birds, fish, and humans. In homing pigeons, he and scientists at Oxford are now trying to find out how flocks of birds can "pool their knowledge to navigate as a group." Mann has also published a paper about the best strategy for a game of Battleship—a problem that, compared to navigating home from an unfamiliar place, is more suited to a feeble human brain.
Images: top by ZeroOne (via Flickr); bottom, Mann et al.
Mann RP, Armstrong C, Meade J, Freeman R, Biro D, & Guilford T (2014). Landscape complexity influences route-memory formation in navigating pigeons. Biology letters, 10 (1) PMID: 24451267