Hawks prioritise safe landings over saving energy or speed

Researchers at the Oxford Flight Group have found that hawks control their flight to ensure the safest landing conditions when perching, even if it takes longer and is more energetically expensive to do so. Understanding how birds optimize their landing manoeuvres through learning may help in developing small aircraft capable of perching like birds.

Four Harris’ hawks (Drogon, Rhaegal, Ruby, and Toothless) were flown back and forth between two perches whilst wearing tiny retroreflective markers. Their motions were recorded by 20 motion capture cameras positioned around the room, allowing the researchers to reconstruct their flight paths on over 1,500 flights. The team then used computer simulations to understand why the birds chose their particular path to the perch.

Whereas aircraft have the luxury of using a runway for braking after landing, birds must brake before they arrive at the perch. But slowing down to a safe speed while in flight risks stall, leading to a sudden loss of flight control. The researchers discovered that the hawks follow a flight path that slows them down to a safe speed whilst minimizing the distance from the perch at which they stall.

To solve this problem, the hawks dived down towards the ground whilst flapping, before spreading their wings into a gliding posture as they swooped up to the perch. By selecting just the right speed and position from which to swoop up to the perch, the birds were already within grabbing distance of the perch when they stalled, keeping their landings as safe and controllable as possible.

“We found that our birds weren’t optimising either the time or energy spent, so their swooping trajectories were neither the shortest nor cheapest options for getting from A to B. Instead, our birds were reducing the distance from the perch at which they stalled, and were even better at limiting stall than our simplified computer model.”

Lydia France

Hawks break at the last moment before landing

Landing is a critical manoeuvre, and stalling has been the cause of many air accidents. But birds perch safely and make it look easy. Looking at birds and asking how they solve the problem of safe landing might help us find new solutions for our own technologies, including small aircraft capable of perching like birds. This is called bioinspired design.

Understanding how birds learn complex motor tasks like landing might also help improve artificial intelligence (AI). When aircraft engineers use computers to solve the problem of perching using a trial-and-error approach called reinforcement learning, it takes their computers tens of hundreds of hours to find an answer. Yet, hawks find an optimised solution over a handful of flights, showing the massive gap that still exists between natural and artificial intelligence.

“Motion capture technology has allowed us to analyse thousands of flights at a time, tackling questions that we never could have done before. Looking forward, this opens the tantalising possibility of understanding how animals learn complex motor tasks, like learning to fly, and of revolutionising how robotic systems can do the same.”

Graham Taylor

To read more about this research, published in Nature, visit: https://www.nature.com/articles/s41586-022-04861-4