No wonder so many bird species fly in "V" formations: it saves energy.
As they migrate south, these northern bald ibises carefully position themselves in the V formation and flap their wings at precise times. As a result, each bird finds it easier to stay aloft, saving energy for the whole flock.
"They're trying to nick some lift from the bird in front," says Steven Portugal of the Royal Veterinary College in London. His team taught 14 captive-reared birds to follow a powered hang-glider as if it was the leading bird in a V. They also fitted the birds with lightweight monitors that recorded their position, their speed and the timing of every wing flap.
They found that the V formation enables each bird to avoid downdraughts from the one in front, which would otherwise push them earthward. But that's not all. By carefully timing the beats and positions of their wings, trailing birds tap into uplift from narrow vortices of rising air that swirl backwards from the wingtips of the bird in front.
Portugal says models had predicted these effects, but his results are the first evidence. However, it is unclear how much energy the birds save, and why some, especially small songbirds, forsake V formations. "There seems to be a point, maybe around the size of seagulls, when it becomes worth doing it," says Portugal.
Journal reference: Nature, DOI: 10.1038/nature12939
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