The V formations adopted by migrating birds are well known to anyone looking skyward at the end of summer. It is a formation regularly adopted by military and civilian aircraft for its energy saving benefits. Fully understanding how these formations work in nature has intrigued scientists for a long time. A new study into the biomechanics of how this works has made it onto the cover of this month’s Nature magazine – and it turns out that making use of these benefits is much harder for birds than for aircraft!
The way that wings generate lift is all to do with how they interact with the air they move through. Air moves faster over the top surface of the wing than it does over the bottom surface, creating a net circular flow relative to the stationary air the wing is moving through. These circular vortices are shed at the tips of wing forming a tube of spinning air that extends back from each wing tip (see the image below). The air on the inside of this tube (directly behind the wing) is moving down, a so-called downwash. The outside edge of the tube is formed of upwards-moving air creating an upwash. By carefully positioning a wingtip in this upwash region, a following bird or plane is able to ride this upward moving air, thereby reducing the amount of energy needed to fly.
A skilled pilot can make use of this to save fuel on long flights. Birds, however, have another obstacle to overcome: the flapping of their wings. As the lead bird flaps, the tip vortex oscillates up and down. To make optimal use of the tip vortex, a following bird would have to move its wings with a precise phase shift so that its wing tip follows the up and down motion of the tip vortex. Several theoretical studies have predicted these requirements but until now it had not been possible to test the plausibility of this on birds in flight.
Portugal et al.  used specially designed data loggers that measured both the positions of birds in a flock and when they beat their wings. The important thing is that these data loggers have to be small (23g) to avoid impacting on the birds’ flight, and that meant they had no means of transmitting the data to researchers. The researchers had to collect them. Obviously it would have been no good to put them on a random set of birds, only to watch them fly off into the sunset, with no idea of where they might end up!
Instead the researchers used a flock of northern bald ibises that had been part of a conservation programme, and had been taught by conservationists to follow certain migration routes. The team therefore knew where the birds would land to rest and could pick up their data loggers there.
Amazingly, the data collected showed that the birds were able to position themselves and to flap their wings with almost precisely the correct phase shift to make best use of the tip vortex of the bird in front. This ability was far beyond the expectations of the researchers. Even more impressively, the birds were able to adapt their flapping when changing position within the V. This often requires them to pass behind another bird, into the downwash generated by the bird in front. In order to minimise the effect of this, the trailing bird flies in direct anti-phase with the leader as it passes behind it, returning to the optimal phase shift once it can make use of the tip vortex again.
Such dynamic changes in flapping to maintain efficient energy usage demonstrate an incredible adaptability and raise many questions. How are the birds able to find these optimal flight patterns? Is it instinctive behaviour or do the birds learn to fly this way because it feels easier? How much benefit do these techniques have on the birds themselves? Hopefully we can look at those beautiful V formations with a whole new sense of wonder this year.
1. 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
All images from Wikipedia Commons
By Matthew Evans- a second year PhD student in the lab of Dr Richard Morris