We now know how kestrels stay perfectly still while hovering

Kestrels have been trained to fly inside a wind tunnel to reveal how they can hover in turbulent winds while keeping their heads almost perfectly still.

Training the two captive-bred nankeen kestrels (Falco cenchroides) took three years, says Abdulghani Mohamed at RMIT University in Melbourne, Australia. Adding to the difficulty, each bird was wearing 52 reflective markers so they could be filmed to create the same 3D models that are used in computer-generated imagery.

The hovering behaviour is critical to the birds as it allows them to keep their heads perfectly still to focus on prey on the ground. “It’s a hunting behaviour and their life depends on it,” says Mohamed. “They have an amazing level of steadiness.”

In the wind tunnel, researchers could create repeatable and measurable turbulence conditions, enabling them to study the specific movements of the kestrels and what they respond to. Altogether, the team recorded 58 hours of hovering over five years.

The researchers found that the birds’ hovering was so stable that their heads didn’t move beyond 2 millimetres in any direction.

“It’s a magnificent scene every time I watch the birds hovering in the wind tunnel,” says Mohamed. “To see them so effortlessly and gracefully hovering is just spectacular.”

Unlike conventional aircraft, which have fixed wings and limited control over their surface area, kestrels have morphing wings that are able to almost instantly adjust their surface area. This is the crucial factor in enabling them to achieve stillness, says Mohamed.

The kestrels did this most successfully by deftly moving their wrists and subtly extending and retracting their elbows. Their tails also played a crucial role in providing extra stability, says Mohamed.

Based on the findings, the team has now constructed a prototype for an unpowered drone to test in the wind tunnel.

“It’s very difficult to do an exact replica of all the anatomy of a kestrel, so we have distilled our findings down to the main contributors to stability – wrist and tail movements – and built a robotic version of a kestrel,” says Mohamed.

The artificial kestrel is currently being tested, with results expected later in the year.