How Do Birds Avoid Crashes?
The secret of how birds zip flawlessly through narrow spaces without crashing into obstacles has been unlocked by Australian scientists.
Their discovery could be used to design ‘bird-safe’ buildings and windmills, and improve the versatility of pilotless aircraft.
Researchers at The Vision Centre have found that birds weave rapidly and safely through dense forests and narrow corridors by using their eyes to sense the speed of background image flow on both sides and adjust their flight according to it.
“As animals travel forward, things that are close seem to speed by, and things that are farther away seem to travel more slowly,” says Professor Mandyam Srinivasan from The Vision Centre and The University of Queensland, whose team made the discovery. “It’s the same for birds. We found that they try to achieve a safe ‘balance’ by ensuring that the background images are passing at the same speed in both eyes.
“This means that if the bird flies closer to obstacles on one side, the near eye will see things passing by faster while those seen by its other eye will pass more slowly. This imbalance prompts the bird to veer away to even out the speed of image flow in both eyes.”
To find out how birds navigate through narrow passages and away from danger, the research group trained budgerigars to fly along a corridor with walls lined with horizontal or vertical stripes, says Dr Partha Bhagavatula of The Vision Centre and The National Vision Research Institute.
“We found that birds fly the fastest when both walls are lined horizontal stripes, because the stripes are parallel to the bird’s flight direction, and the birds don’t ‘see’ a strong image flow in the background,” Dr Bhagavatula says. “But when both walls have vertical stripes, birds slow down significantly due to the strong image motion, which shows that birds also regulate their flight speed according to what they see.”
When the walls were set up with different orientations – one with vertical and the other with horizontal stripes, the group found that birds flew significantly closer to the horizontal stripes.
“As vertical stripes project a stronger image flow to their corresponding eye, they veer away to restore the balance between the flows experienced by their two eyes,” says Dr Bhagavatula. “This was also demonstrated when one wall was left completely blank. Then the birds flew very close to, and occasionally collided with, the blank side.”
Prof. Srinivasan says that flight behavior in birds is very similar to insects such as honeybees, bumblebees and flies: “This suggests that this principle of visual guidance may be shared by all day-active flying animals.
“Furthermore we believe these findings can contribute to the technology of guiding unmanned aerial vehicles where aircraft have to fly through obstacles in cluttered environments, or through canyons and gorges, or under bridges.
“Another potential application is the design of urban structures that are more bird-friendly to minimize the risk of bird fatalities through collisions with window panes. We can also think about decorating windmill blades with patterns that generate motion signals to repel birds.”
The group’s paper “Optic flow cues guide flight in birds” by Partha Bhagavatula, Charles Claudianos, Michael Ibbotson and Mandyam Srinivasan has been published in the latest issue of Current Biology.
Part of the research was carried out at the Research School of Biology at The Australian National University.
The Vision Centre is funded by the Australian Research Council as the ARC Centre of Excellence in Vision Science.
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