Anecdotal evidence from breeders of racing pigeons as well as initial aerial tracking studies together suggested that pigeons may follow roadways and use highway landmarks as turning points in their flight. However, the challenge of accurately tracking the birds stood in the way of solid quantitative analysis. In the new work, miniaturized GPS "flight-loggers," which pigeons carried on their backs, allowed researchers a clear and reliable picture of the birds' flight paths.
Over three years, the researchers analyzed more than 200 flight paths of 20-80 km in length made by pigeons travelling toward their home loft from numerous release sites located in the general vicinity of Rome, Italy. They found that, when released from familiar sites, pigeons with homing experience were significantly attracted to highways and a railway track running in the approximate directions home. When these structures began to veer significantly from the beeline to the loft, some birds tended to break away and head in a more homeward direction, but others took a detour by following the highway until a main junction, at which point they followed a valley road in the direction of the loft.
Overall, the degree to which the birds appeared to follow roadways was strongest in the early and middle sections of their homeward journeys, when, the researchers suspect, roads serve to stabilize the birds' innate compass course. As they approached the loft, the birds appeared to switch strategies and rely more on topographical points as bona fide "road map" components. The birds may use these features in a cognitive strategy that helps them locate their loft in their more immediate home area.
The researchers found that, although following roadways in some cases represented significant digressions from the beeline home, in general the early benefit of the roadways in keeping the birds on course seemed to compensate for the added flight distance. In fact, the birds appeared to prefer to follow roadways once they became experienced with the release sites; individuals who made three or more journeys home showed an increasing tendency to follow highways. The authors speculate that the preference for roadway following could form because easier navigation may allow brain processing-time and power to be directed to other tasks, such as watching for predators.
The pigeons appeared to be attracted to the SS Aurelia, Italy's old coastal highway, to a greater extent than the newer and larger highway A12 or the railway. The SS Aurelia traces the ancient imperial Via Aurelia, which was begun in 241 BC and connected Rome to what is now southern France.
It is unclear the extent to which pigeons or other birds may employ strategies akin to road following in non-urban areas, but the authors point out their agreement with previous assertions that homing pigeons appear to have "a remarkable ability to shift from one homing strategy to another," and they urge that their new findings be reasonably interpreted.
Hans-Peter Lipp, Alexei L. Vyssotski, David P. Wolfer, Sophie Renaudineau, Maria Savini, Gerhard Tröster, and Giacomo Dell'Omo: "Pigeon Homing along Highways and Exits"
Publishing in Current Biology, Volume 14, Number 14, 27 July 2004, pages 1239-1249.