The new research reported in the journal Nature shows for the first time that birds precisely time when they flap their wings and position themselves in aerodynamic optimal positions, to maximize the capture of upwash, or ‘good air’, throughout the entire flap cycle, while avoiding areas of downwash or ‘bad air’.
The V-shaped flight formation allows each bird to receive lift from the wingtip vortex of the bird in front of it, saving energy and greatly extending the range of a flock of birds over that of a bird flying alone. Image credit: Russell Link / Washington Department of Fish and Wildlife.
“The distinctive V-formation of bird flocks has long intrigued researchers and continues to attract both scientific and popular attention, however a definitive account of the aerodynamic implications of these formations has remained elusive until now,” said study lead author Dr Steve Portugal from the University of London.
“The intricate mechanisms involved in V-formation flight indicate remarkable awareness and ability of birds to respond to the wingpath of nearby flock-mates. Birds in V-formation seem to have developed complex phasing strategies to cope with the dynamic wakes produced by flapping wings.”
The 14 juvenile Northern bald ibises (Geronticus eremita) used in this study were hand-reared at Vienna Zoo, Austria.
The birds were trained to follow a micro-light ‘mother-ship’ to teach them their historic migration routes to wintering grounds in Italy. Normally they would learn this from adult birds, and without this help, the birds would not thrive. The birds are currently in Tuscany and the team hopes they will remember the way to what should be their breeding grounds in Salzburg later this year, without the help of the micro-light this time.
The mechanisms that the birds use is achieved through spatial phasing of wing beats when flying in a spanwise V-position, creating wing-tip path coherence between individuals which will maximize upwash capture throughout the entire flap cycle. When flying in a streamwise position, birds exhibit spatial anti-phasing of their wing beats, creating no wing-tip path coherence and avoiding regions of detrimental downwash. Such a mechanism would be available specifically to flapping formation flight.
The study is the first to collect data from free-flying birds and was made possible by the logging devices custom-built at the Royal Veterinary College.
The light-weight, synchronized, GPS and inertial measurement devices, recorded within up to 30 cm accuracy where a bird was within the flock, its speed, and when and how hard it flapped its wings. The precision of the measurements enabled the aerodynamic interactions of the birds to be studied at a level and complexity for the first time.
“This is a fascinating piece of research, providing a scientific answer to a question that I suspect most people have asked themselves – why do birds fly in formation? The results will prove useful in a variety of fields for example aerodynamics and manufacturing,” said Prof David Delpy of the Engineering and Physical Sciences Research Council in Swindon, UK, who was not involved in the study.
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Portugal SJ et al. 2014. Upwash exploitation and downwash avoidance by flap phasing in ibis formation flight. Nature 505, 399–402; doi: 10.1038/nature12939
