FORMATION FLIGHT
Many birds fly in formations (Figs 1 and 2). These include V shaped formations, but also lines, and echelons. The phenomenon is most often noted in larger birds and smaller birds tend to fly in much less structured and loose aggregations (Fig 3). The function of formation flying has been a matter of debate for over 100 years. Two dominant hypotheses have been developed. The first is that birds fly in formations as a method of orienting themselves. By flying closely adjacent to another individual this allows the following individual to orient to a feeding or roosting site more easily. Alternatively it is suggested that because wings shed vorticies by strategically locating themselves behind and to the side of another bird, a trailing bird can gain lift and thus reduce its own energy demands for flying.
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Fig 1 |
Fig 2 |
Fig 3 |
The energy saving hypothesis makes very specific predictions about the optimal location in which birds should position themselves to make maximum advantage of the trailing vortex. This involves a slight overlap of wing tips. The orientation model does not make such specific predictions and is thus harder to refute. Indeed the function of the behaviour may in any case be dual - involving benefits in both energy saving and orientation. Demonstrating that the behaviour has energetic benefits does not therefore preclude the possibility that orientation benefits occur. Moreover demonstrating no energetic benefits for the behaviour does not by default provide support for the orientation hypothesis.
Our work has involved photographing skeins of geese from below and reconstructing the positions of the animals relative to each other and then asking how close the positioning of the birds equates to the position which maximises energy saving. By using a model of the impact of position on the assisted lift that can be acheived we have also attempted to quantify the energetic benefit that accrues to a bird when it is flying in formation. We have studied two types of geese, the small pink footed goose () and the much larger grey lag goose (Anser anser).
We have found that the birds roughly locate themselves around the optimal position for energy saving, but their ability to sustain this position is limited. Consequently the energy savings from formation flying are relatively trivial in the small pink footed goose at around 2.5% of total flight costs. In contrast the savings for grey lag geese were substantially higher. This effect of size on the realised savings in formation flight may suggest that larger animals find it easier to fly in the most favourable position. Several factors may influence this effect but an importat factor is probably wing beat frequency. Smaller birds beat their wings faster. Accordingly the exact location of the vortex may be difficult to track accurately (The predicted locations assume no flapping and positional stability of the vortex). In addition larger birds may be less affected by small gusts of wind.
In combination then these effects mean the realised savings from formation flight get lower and lower as the birds get smaller, until at some point the benefits of formaiton flying become impercetible. At this point there would be no evolutonary pressure for the trait to evolve. This probably explains why small birds do not fly in formations like much larger birds tend to.