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Animal behaviour studies

Study 3: V Flight formation in migratory Ibises

The characteristic V formation flight of birds has fascinated scientists for centuries. One of the main theories has been that birds are attempting to conserve energy by taking advantage of the upwash vortex fields created by the wings of the birds in front. For the first time, using data collection technology developed within our CARDyAL project, we have been able to identify the aerodynamic mechanism by which birds can capture this upwash.

Our study, published in Nature, used data from free-flying migrating birds to reveal how birds flying in V formation can utilise areas of beneficial upwash while avoiding regions of detrimental downwash.

What did the study discover?

Ibises in flightThe first key finding was that individuals positioned themselves in aerodynamically optimum positions - predicted by fixed-wing (like an aeroplane) aerodynamic theory. Given that the birds are flapping their wings, unlike a plane, this was quite surprising.

It was how and when the birds flapped their wings, however, which provided the most interesting result and revealed the mechanism by which birds in a V formation can make use of upwash.

This is achieved firstly through spatial phasing of wing beats when flying in a spanwise (V) position, creating wing-tip path coherence between individuals which will maximise upwash capture throughout the entire flap cycle.

Secondly, when flying in a streamwise (behind) position, birds exhibit spatial anti-phasing of their wing beats, creating no wing-tip path coherence and avoiding regions of detrimental downwash.

Birds flying in a V have these phasing strategies to cope with the dynamic wakes produced by flapping wings. Such wing-tip path coherence and wing-beat phasing had formerly been suggested in theoretical engineering models and never before recorded in birds; it was previously assumed to be too complex a feat to perform.

Such intricate mechanisms suggests an amazing awareness of the spatial wake structures of nearby flock mates, and imply the birds have a remarkable ability either to sense or predict it.

How was the study carried out?

The research on V formation flight, led by Dr Steven Portugal, focused on migratory flights of the critically endangered Northern bald Ibis - known in Austria as the Waldrapp ibis - (Geronticus eremita) pictured below.

Waldrapp Ibis

We measured the position of individuals within a V formation flock, and identified when each bird flapped its wings. This opportunity was provided by a ground-breaking conservation organisation, Waldrappteam, who are re-introducing the Northern bald ibis into its former breeding range. The ibises were hatched at Vienna Zoo, and immediately imprinted onto human foster parents who would lead them on the migration journey.

ibises with microlightFoster parents with ibises

Above: ibises with foster parents
Right: flying with the microlight



It is crucial that the ibis learn what would have been their historic migration routes from breeding grounds in Austria, to their wintering quarters in Italy. The birds learn the route by being trained to follow a microlight paraplane containing one of the foster parents.

We were able to join forces with the Waldrappteam to fit 14 Northern bald ibises with our data loggers, to record the aerodynamic interactions between individuals in a V throughout a migratory flight across Austria. The map below shows a section of the migration route.

Map of ibis migration route

To find out more about the reintroduction project and the ibises, see www.waldrapp.eu/waldrappteam.

Collecting position and movement data from free-flying birds: a technological challenge

Logger being fitted to ibisWe developed special data loggers for this study, consisting of a high-accuracy GPS recording the position of the bird five times a second, synchronised with an accelerometer to record the bird's movements. The accelerometer data enabled us to identify when each bird initiated a flap. The loggers also needed to be light enough not to affect the birds' flight characteristics - they weighed 27g, less than 5% of the smallest bird's mass - but with enough battery life for each day's flight. Over 180,000 wing beats were analysed.

The ibises wore dummy loggers on their backs for a month or so during training flights to prepare them for the real data loggers that they would be equipped with for the migratory flights.

This is the first time such measurements have been made on free flying birds undertaking long migratory flights.

Publication

Portugal, S.J., Hubel, T.Y., Fritz, J., Heese, S., Trobe, D., Voelkl, B., Hailes, S., Wilson, A.M. & Usherwood, J.R. (2014). Upwash exploitation and downwash avoidance by flap phasing in ibis formation flight.
Nature 505, 399-402. doi:10.1038/nature12939

Outcomes

In this study we have explored aspects of V formation flocking that were not previously possible. We have investigated the positive aerodynamic interactions that take place between individuals within a V flock, and discovered the flap-phasing strategies birds use to maximise upwash capture and minimise the effects of downwash. It allows comparisons to be made between V formation flight and our earlier work on cluster flocking.

The findings of such investigations have many potential applications include collective flying of airplanes, unmanned aerial vehicles and initiating a desired motion pattern in crowds or groups of animals.

Acknowledgements

We thank the EPSRC, BBSRC and Wellcome Trust for funding the study and the Waldrapp team for their generous collaboration. All images by kind permission and copyright Markus Unsöld.

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