Kestrels have been trained to fly in a wind tunnel, revealing how they can hover in turbulent air while keeping their heads almost completely still.
Two Chinese kestrels (Falco cenchroidesIt took three years to complete, says Abdulgani Mohammed of RMIT University in Melbourne, Australia. What made it even more challenging was that each bird was tagged with 52 reflective markers and photographed to create the same 3-D models used in the computer-generated images.
Hovering is crucial for birds, allowing them to keep their heads completely still while they focus on prey on the ground. “It’s a hunting behavior, and the bird’s life depends on it,” Mohammed says. “They have an incredible sense of stability.”
In the wind tunnel, the researchers were able to create repeatable and measurable turbulent conditions that allowed them to study the specific movements and reactions of the kestrels. In total, the team recorded 58 hours of hovering over a period of five years.
The researchers found that the birds’ hovering motion was extremely stable, with their heads never moving more than two millimeters in any direction.
“It’s an amazing sight to see every time I see the birds hovering in the wind tunnel,” Mohammed says. “It’s truly amazing to see how effortlessly and gracefully they hover.”
A Chinese kestrel flying in a wind tunnel
RMIT University
Unlike conventional aircraft with fixed wings and limited control over wing area, kestrels have transformable wings that can adjust their wing area almost instantly, which is key to them being able to remain stationary, Mohammed said.
The kestrels did this best by using clever wrist movements and subtle extensions and retractions of their elbows, and their tails also played a key role in providing stability, Mohammed said.
Based on their findings, the team is now building an unpowered drone prototype for testing in a wind tunnel.
“It’s very difficult to accurately recreate all of the kestrel’s anatomy, so we narrowed our findings down to the wrist and tail movements that contribute most to stability, and built a robotic version of the kestrel,” Mohammed says.
The artificial kestrel is currently being tested, with results expected later this year.
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