Geese glide swiftly through the air in a V-formation that provides great aerodynamic benefits, including decreased energy requirements. Pigeons, on the other hand, fly in busy cluster flocks -- taking sharp, banked turns and flapping their wings rapidly -- which, it seems, takes a great deal more energy than flying solo, according to new research published this week in Nature.
The finding suggests that pigeons, and other cluster flocking birds, fly in flocks for reasons other than energy efficiency.
"It's very interesting," said Geoff Spedding, chairman of the Department of Aerospace and Mechanical Engineering at the University of Southern California, who was not involved in the research. "As far as I know, nobody has had the instrumentation before to measure things such as wing beat frequency in flocking animals."
Jim Usherwood and a team at the Royal Veterinary College at the University of London tracked 18 racing pigeons during 7 flights over 2 days. The animals were equipped with mini-backpacks sporting GPS systems, accelerometers, and gyroscopes, which measure position, motions, and changes in orientation, respectively. The equipment recorded that, within a flock, the pigeons made tight, banked turns like aircrafts do, which quadrupled power requirements, and that they flapped their wings faster, also increasing energy expenditure. In fact, the closer they were to neighboring birds, the faster they flapped.
Flocking, therefore, comes at an energetic cost to pigeons, the authors concluded, and is not the reason they fly together. Because these birds are well-fed racing pigeons free to fly whenever they desire, it is possible, in this case, the birds were flying for the sake of exercise. "Presumably being a fit, good flier has all sorts of selective benefits," said Usherwood, and racing pigeons have been specifically bred to be athletic and fast.
But birds may also flock to gain the advantages of collective navigation, greater safety from predators, or for social benefits. "Energetics isn't everything," said Usherwood.
"There are probably many overlapping reasons for flocks of various kinds," said Spedding, who wrote an accompanying News & Views article in Nature. It possible that some flocks form simply as a consequence of social behavior: Some birds just tend to stay close to each other -- on the ground and in the air.
Path of a single pigeon, and relative positions of other pigeons, for a single flight. Playing at double realtime. Courtesy of Jim Usherwood, Structure and Motion Lab, The Royal Veterinary College. From thescientistllc on Vimeo.
J. Usherwood, et al., “Flying in a flock comes at a cost in pigeons,” Nature, 474:494-7, 2011.