When a goose comes in to land, sometimes, if you are lucky, you will see it do a “whiffle”. Approaching the water, it twists and turns to spill air from its wings — often ending up completely upside down in the process.
What is perhaps even more extraordinary than an upside-down goose is that throughout its contortions the goose’s head stays still and upright.
Now scientists have investigated the ability of geese and swans to keep their heads still while flapping and being buffeted in the wind, and have found that they appear to have a neck mechanism like a car suspension.
“Very little was known about how birds stabilise their heads,” said David Lentink, of Stanford University. “You can imagine that without vision it is very difficult to navigate, and for vision to work you need to stabilise for wind and flapping. How do you remove jitter from vision?” He thought something clever was happening.
“They can do exquisite things with their neck. I got hooked on this through this behaviour of whiffling. I started to realise, there must be a lot of stuff going on in that neck.”
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Many animals, including humans, have some form of automatic head stabilisation that they employ while moving. This makes life a lot easier for our visual systems — instead of trying to cope with continually processing jerky images, the brain can save space for more important tasks.
For the paper, published in the Journal of the Royal Society Interface, scientists observed videos of swans in flight to see how their head movements correlated to the flapping and buffeting experienced by the rest of their bodies. They found that the neck worked extremely effectively to “damp” the body’s motion. The head is stabilised to such an extent that its movements are much less pronounced than the body.
Precisely how the neck does this was very difficult to determine. A goose or swan’s neck has three times the number of vertebrae as human’s, and 200 muscles on either side.
He and his colleagues showed that they did not need to understand precisely what went on in the neck to see what was happening. The neck’s behaviour instead correlated closely to that of a mass-spring-damper system, in which a load is attached to a spring.
They believe that the bird just “tunes” its internal spring, leaving enough brainpower for more complex behaviour such as whiffling.
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“On a wingbeat-by-wingbeat basis the only thing it needs to do is tune the neck for stiffness. It’s really like car suspension: our car springs absorb little bumps and only with bigger bumps do we need advanced active control systems.”
Professor Lentink, a mechanical engineer, hopes the research will help to create stable cameras for “insect” drones, powered by flapping.