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One February morning in Delhi was the first time I did something I’ve done a few times since. I rode my bicycle to one end of the runway at the airport and stood there for a couple of hours, watching planes land and take off. I remember that very clearly, but not really because of the planes. Riding home, my fingers got so cold that I feared they would fall off. Seriously.

Still, the planes. Both take-offs and landings were exciting, but the landings were far more intriguing. At some point afterward, I began wondering - when they descend from the sky, why do these flying machines need a long runway to slow to a stop? After all, their very creation was inspired by birds. With few exceptions, birds are able to transition from flight to perching on a branch in an instant. Admittedly, they are much smaller and slower than aircraft, and thus easier to stop. Even so, what is it about the flight of birds? Do they have some special mechanism in their wings and muscles that we have not (yet) been able to replicate in our planes?

True, different birds behave differently while coming to a halt. Watch flamingos swoop in to land in shallow water, and it’s almost as if they run along the surface for a few steps before settling - reminiscent of planes using a runway. But there are small birds like sparrows or flycatchers, which seem to hover for moment right before sinking onto a twig. Or larger ones like hawks and kites, that will actually swoop up from below to perch.

I know this partly from watching plenty of birds on the trees near my home. But also because, as you have probably guessed, there are scientists who have studied the flight of birds. In particular, how they land. A recent paper in Nature, for example, begins thus: “Perching at speed is among the most demanding flight behaviours that birds perform and is beyond the capability of most autonomous vehicles" ("Optimization of avian perching manoeuvres", Graham K. Taylor et al, 29 June 2022.

Beyond the capability of, for example, planes.

The paper says researchers tracked the flight paths of four hawks - not once or twice, but nearly 1700 times over distances of 5, 7, 9 and 12 metres. Three were young males, “initially naïve to the task"; the fourth was “an experienced adult female." They flew these distances tempted by food the scientists made available at flight’s end.

And what did they find? For their first few flights, the naïve birds flapped their wings and flew more or less directly across to the food. But only those first few flights. After that, some innate hawk instinct took over and they followed the technique more experienced birds - like the adult female - used. It went something like this:

The hawk leaps from where it is stationed, a post about 1.25m tall. It dives forward and downward, using “several powerful wingbeats" to do so. A little over halfway to the food, it has sunk to as low as just 35cm off the floor. But from that nadir, it climbs upward to the food. Its wings do hardly anything during the climb apart from limited moves to correct and control the flight. This “unpowered climb" is essentially a glide, using the momentum of the earlier dive to move forward and upward. As the bird nears the perch, it switches mid-flight to a position in which its body is nearly upright, its wings are outstretched and its feet are held out in front to grab the perch.

Thus lands the hawk. Seen from the side, its flight path is a shallow “U", the swoop of its trajectory unmistakable. But why this ‘U"?

The researchers report that after a session of these experimental flights, the birds were “usually panting visibly." You might conclude that while the hawks fly at relatively slow speeds over these relatively short distances, those powerful wingbeats use up a lot of energy. The slower a bird’s flight, after all, the more it needs to flap its wings, the less it will glide. This is why the hawk dives at the start, you’d think, using gravity to increase speed. And while flapping does need substantial energy, the subsequent gliding climb means that overall, this U-shaped path uses less energy and even time than if the hawk flew directly to the food, beating its wings all the while just to stay in the air.

In other words, hawks seek to minimize the energy used and time taken on a flight. This is the lesson experience quickly teaches the birds.

Or is it? What the scientists came to understand over those 1600 flights was slightly more subtle. The hawk’s flight path was not necessarily one that made the best use of energy and time, but one that gave it the best chance of perching in a controlled, stable way. Every time. In fact, when the bird switches to that nearly upright position with its wings outstretched, it has practically stopped in mid-flight, in the air - and that’s what lets it make a gentle, precise landing.

In effect, what the hawk is executing is a “stall" - a well-known danger for pilots. Raise the nose too much at too low a speed, and the plane stalls and falls out of the sky. 

The hawk seeks a stall too. But if it happens too early, it falls and misses the perch. Too late, and it crashes at speed into the perch. The hawk’s goal, then, is to minimize the distance of its stall - to postpone the stall as late as possible - to allow for that spot-on landing.

What hawks and other birds have, then, is the ability to change the shape of their wings mid-flight. That’s how they can carry out these precise manoeuvres with such apparent ease.

And until our flying machines can do that, we will need those long runways.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.

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