Freeloading: Secret of bird flight
Bird flight is endlessly fascinating. How do they navigate? How do they get enough energy for long migrations? And why do so many birds fly in a vee-formation?
This week, we get confirmation that the vee formation eases flight because the birds can ride updrafts created in the vortex — whirling wind — behind the leading bird.
Are the birds solving complex equations of fluid dynamics in their bird brains? Not. Those brains, intimately attuned to flight, apparently recognize a good deal and adjust the position as necessary.
The critically endangered bird under study, the northern bald ibis, nests in colonies and eats small prey like lizards and insects. The wild population, in Morocco, is down to about 500.
The new study piggy-backed on Waldrappteam, a European reintroduction project that is teaching new migration routes to ibises in Europe. The group’s follow-the-ultralight approach echoes a whooping crane migration project in North America.
The scientists strapped a 23-gram package that used GPS and a gyroscope to measure acceleration and location to 14 birds. Data from those instruments supported the study’s analysis of the spatial relationship between a bird and the one immediately ahead of it.
Into the vortex
When bird flap their wings, they create a vortex, which resembles a micro-tornado flipped sideways. Although the position of the trailing bird varied from second to second, they did spend most of their time with their wings in the updraft of the vortex.
Although the researchers did not measure effort, the bird’s careful positioning is expected to ease flight, says article author James Usherwood. “They want their wing to be where the air is going up; they want to follow wherever the uplift is.” Usherwood studies animal biomechanics at the Royal Veterinary College in the United Kingdom.
To the birds, the updraft provides free energy, much like the gift of a tailwind to a bike rider. And the updraft is not just important when the trailing bird’s wing is moving down in the power stroke, says Usherwood. “They probably benefit from wake support all through the flapping cycle.”
On the flip side, staying in the updraft helps the birds shun the downdraft section of the vortex.
Although a wake in air is invisible, it’s similar to the wake trailing a speedboat — a rotating disturbance with its long axis more or less parallel to the boat’s/bird’s direction of motion.
What’s it worth?
It’s likely — but unproven — that the vee formation chosen by Canada geese and many other migrating birds is driven by the same hunt for the updraft, Usherwood says.
How much does riding the vortex help the trailing bird? We asked Usherwood to do some arm-waving, but he wisely retreated, telling us, “The energetic benefit is as yet undetermined.” Our understanding how much energy a flapping bird requires to fly “is very poor,” he continued. “We could pretend they are fixed-wing aircraft” and apply aerospace engineering calculations, but that’s a “very unhappy” solution.
In assessing the relative energy output, the angle of the formation matters, Usherwood says. “According to aerodynamic theory, it’s possible that the leader could be helped by the ones behind. In a shallow vee, it can spread the benefit across all of the birds, or benefit the lead bird.”
But how do the birds choose that aerodynamic sweet spot? The answer is, Usherwood, “As yet undetermined, but possibly more tractable.” People quickly adapt when asked to walk or run with a peculiar gait, he says. “That gives them an unnatural situation, but they can move something else to change their energetics.”
In other words, birds aren’t making intricate aerodynamic calculation on the fly, he says, “but it does not take much calculation to get to, ‘It’s a nice thing if [the leading bird] flaps and then I flap.’”
– David J. Tenenbaum