To soar or to flap?

TUESDAY, 21 DECEMBER 2010

Evidence gathered from large birds has shown that ‘soaring-gliding’ flight is much more energy efficient than flapping their wings. Until recently, however, it was thought that for smaller species soaring-gliding flight would not be advantageous compared to wing-flapping. A smaller musculature and wing structure was thought to mean that their flight speeds when soaring-gliding would be much less than when they were flapping their wings, and so a prolonged flight time would incur a high cost in terms of fitness. Scientists therefore believed that birds weighing below 0.9kg would always preferentially fly by flapping their wings when migrating.

An international team of researchers has disproved this assumption by tracking migrating European bee-eaters, using radio transmitters attached to the birds’ backs. European bee-eaters weigh approximately 55g, and yet it was found that they often switched between flapping and soaring-gliding during their journey. Using heart rate as an indication of energy consumption, soaring-gliding was also found to allow much greater energy savings for these smaller birds than for larger species. Herring-gulls weighing 0.9kg, for example, are known to use 30% more energy when soaring-gliding than when they are resting, whereas the bee-eaters used the same amount of energy when soaring-gliding as when they were at rest in their nests.

The results of this study not only show that small birds, surprisingly, tend to soar-glide when migrating, but also indicate that soaring-gliding is even more energetically favourable for smaller birds than for larger species.

Written by Katy Wei