How can a Kestrel hover in the wind?

Question

Kestrels are birds of prey commonly found in Europe, Asia, Africa, and the North America. They belong to the falcon family but have a unique ability to hover in the air. You can find a whole bunch of videos (See 1,2,3,4, for example) about these fascinating creatures if you search "Kestrel hunting."

(You can click the images below to see the videos)

Rearview : Video from wildaboutimages (link here)

Video from viralhog (link here)

Side view in slow motion: Video from wildaboutimages (link here)

While I admire how they stabilize their head, I am fascinated by their ability to remain still in the air. Note that the bird doesn't have any external support and doesn't flap its wings during this process. There is no horizontal displacement even though there is a reasonably strong wind flow (enough to support its weight).

Why doesn't the bird get thrown backward like, say, a paper plane would in the wind?

While it could be possible that the movement is so small for us to see, watching and rewatching the video makes me think otherwise. Did the birds finally manage to get rid of drag, or is this some very delicate balancing of forces?

It should also be noted that this behavior is not limited to Kestrels or even birds. See this video of a barn owl hunting, for instance (not as impressive, but worth mentioning.), or this video where a hang glider gracefully hovers in the wind.

it’s actually possible to fly backwards if the wind is right… — ZeroTheHero, Sep 19 '21 at 16:52
A non-obvious but important fact is that when you're flying, the ground doesn't matter. What matters is your relationship to the air moving around you, and it's the relationship between the kestrel and the air moving around it that leaves it stationary wrt. the ground. — Williham Totland, Sep 19 '21 at 20:48
Kestrels are not primarily American birds. There is one American species, other species are found in Europe, Asia, Australia and Africa. They probably spread from Africa about 5mya. Indeed some think that the American kestrel is actually a Hobby or another Falcon, and not a "true" kestrel at all. — James K, Sep 19 '21 at 21:18
Notice that the birds are constantly adjusting their wings and tail in order to perfectly match their gliding velocity with the wind. — Wossname, Sep 20 '21 at 00:22
Not only can many other birds - notably seagulls, but many others - do this, hang gliders can too. You just need a rising air current, as when wind hits a steep slope. Though rather than hovering in one spot, you generally see birds travelling along the slope without moving their wings. The rising air allows them to search a large area for food with minimal effort. — jamesqf, Sep 20 '21 at 03:33
Kestrels do flap their wings to hover when they need to. They can hover in virtually still air on cloudy days when there are no thermals. They don't need a rising air current. Of course they prefer to hunt in conditions where they don't need to flap their wings, to conserve energy. — alephzero, Sep 20 '21 at 13:25
@alephzero Yes, I noticed that too. But I asked explicitly about hovering without flapping wings because: 1) It's counterintuitive,2) It looks incredible. — AlphaLife, Sep 20 '21 at 13:39
As @alephzero says, kestrels are known for hovering one moving wings. Many large raptors soar on thermals and other updraughts. But some of the most impressive gliding into the wind can be seen around sea cliffs - gull species (especially kittiwake), and corvids (jackdaws), etc. just hang on the wind by adjusting their lift/drag ratio — Chris H, Sep 21 '21 at 12:52
Notably, hummingbirds are the avian masters of powered hovering—I believe they’re the only ones capable of flying directly in any direction, including backwards. How this is achieved is more obvious, though—they are very much not still while they’re doing it. — KRyan, Sep 21 '21 at 14:09
Imagine there is a conveyor belt on an angle. Imagine you are on a bicycle riding down the conveyor belt. Imagine the conveyor belt is rolling in the upwards direction (a static object placed on it is carried up to the top). Now imagine applying the brakes on the bicycle just sufficiently so that you're getting neither nearer to the bottom nor further away from the bottom. You can stay that way all day (until you wear the brakes out :D ), always rolling "down hill".. That's only the same as what the bird is doing; balancing all the forces — Caius Jard, Sep 21 '21 at 17:20
Can you clarify whether this about kestrels in particular, or birds in general? — Robbie Goodwin, Sep 25 '21 at 21:51

score 25 · Accepted Answer · answered Sep 19 '21 at 16:41

25

A free-body diagram for a fixed-wing airfoil takes into account four interactions: weight, thrust, lift, and drag. For an unpowered airfoil, the thrust is zero.

[source]

These are approximately mutually perpendicular, but not quite:

The weight force always points down.
“Drag” is the part of the aerodynamic interaction that’s antiparallel to the motion through the air.
“Lift” is the aerodynamic interaction that’s perpendicular to the motion through the air.
The direction of thrust (in powered flight) depends on the orientation of your engine.

If the motion of the wing through the air is perfectly level, then the lift and drag forces are vertical and horizontal, and constant-velocity motion (including zero-velocity motion, like hovering) is impossible: there’s nothing to oppose the horizontal drag force, so the wing will accelerate in the direction of the drag. Likewise, if the motion of the wing through the air has an upward component, then the horizontal parts of the drag and the lift point in the same direction. But in the illustration, the motion through the air has a slight downward tilt, which means the lift vector has a forward-pointing horizontal component that can in principle cancel out the horizontal part of the drag.

The kestrel is “hovering” by gliding on a very slight updraft, so that its airspeed exactly cancels the wind’s velocity.

Seagulls also hover, and they do so in flocks. When you see a flock of seagulls hovering, they all do so facing the same direction, and tend to hover relatively close to each other. That’s the place where the updraft is the strongest.

answered Sep 19 '21 at 16:41

rob

89,569

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Just to clarify what I think is the most important factor in your answer - are you saying that this hovering motion is impossible if the wind vector is entirely horizontal (or, indeed downwards) - and that the bird must therefore seek the right vector of wind before it can try to hunt? – Lefty Sep 20 '21 at 09:53
Could you explain why the lift force is tilted forward like that? I don't find that trivial. – AlphaLife Sep 20 '21 at 13:24
@AlphaLife lift is generated perpendicular to the airflow, as gliders are falling, the airflow has an upwards component resulting in a forward component to the lift – Alan Birtles Sep 20 '21 at 13:33
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@AlphaLife it's just defined that way. The net aerodynamic force on the flyer acts in an "arbitrary" direction that isn't aligned to anything in particular but can be decomposed into perpendicular components that are useful to consider independently. For a bird holding station in a steady wind it might arguably be more useful to break it into horizontal and vertical components but these will add up to the same force. – Will Sep 20 '21 at 15:02
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@Lefty that is correct. Birds definitely seek out favorable wind conditions, and particularly updrafts, as a matter of conserving their effort. The wind vector doesn't have to be "right", just have a vertical component in the right direction, as the bird will be able to hold station in a variety of updraft intensities by adjusting its trim. Though they might also simply settle for a slowly descending/drifting hover if ideal conditions are insufficiently available. – Will Sep 20 '21 at 15:08
@AlphaLife The “total aerodynamic force” must be vertical, to balance the vertical weight. Decomposition into lift and drag is a convention, and the convention is the use a right-angled coordinate system. One source. – rob Sep 20 '21 at 15:33
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@Will Thanks for explaining. Like the OP, I had always assumed that birds were able to generate lift from a horizontally moving wind, much like a kite, but I never really understood how they might be able to do this without a tether to cancel the resulting horizontal component. It becomes much easier to visualise when you know that there has to be an upward component to the wind itself. – Lefty Sep 20 '21 at 20:00
@Lefty The kite string means the ground is also involved. A system that can interact with both the air and the ground can extract energy in surprising ways. – rob Sep 20 '21 at 23:25
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@Lefty, The bird probably does not have to find the perfect updraft. It probably only needs to find a sufficient updraft. The bird probably can adjust the efficiency of its glide in a manner similar to how the pilot of a heavy aircraft can adjust the glide efficiency by deploying spoilers. – Solomon Slow Sep 21 '21 at 00:42
@rob Excellent video - thank you! – Lefty Sep 21 '21 at 08:02
@SolomonSlow Yes, I never intended to imply the updraft had to be of a particular magnitude, just greater than zero. Then we enter the realm of what evolution is capable of achieving! – Lefty Sep 21 '21 at 08:05

John Hunter · Answer 2 · 2021-09-19T17:04:15.640

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The three main forces acting on the Kestrel are shown by the arrows below (left).

There is the vertical weight, the lift from the air flowing over the wings and the drag force of the wind, (the blue arrow going slightly upwards, as described in Rob's answer).

These three forces must have a resultant of zero, they make a 'triangle of forces', right diagram.

(seagulls do it too)

edited Sep 19 '21 at 17:04

answered Sep 19 '21 at 16:06

John Hunter

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I don't feel like this fully answers the question. The bird has no control over the "vertical weight" force, so it would need to independently control both the "drag force" and "lift force" to cause a net-force of 0. How is this possible just by tilting their wings? – BlueRaja - Danny Pflughoeft Sep 21 '21 at 02:07
The bird can also choose the right conditions, i.e a place where there is sufficient updraft, please see the discussion under the other answers – John Hunter Sep 21 '21 at 22:01
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Without the bit about updraft being required this doesn't answer the question. – Rick Sep 22 '21 at 13:55
@BlueRaja-DannyPflughoeft Here's how you can do it in a glider soaring in an updraft, using the fact that a stable airplane will automatically settle on an attitude in which these forces are in balance: 1) use the stick to adjust the nose up/down pitch until you are flying at a speed that stops your forward motion (nose down -> fly faster) and use the airbrakes to adjust your lift until you are stationary vertically. Birds make both of these adjustments primarily by changing the geometry of their wings (fore/aft movements will affect the pitch, and spreading/folding will affect total lift. ) – sdenham Sep 22 '21 at 16:38

MichaelK · Answer 3 · 2021-09-20T12:34:44.953

Only if there is an up-draft

Yes, you are perfectly correct in that — had the wind been only horizontal along the ground — hovering would have been impossible (without the bird actively adding thrust on its own).

So in all these cases, the bird is using a very slight up-draft to find that motionless state.

The same goes for that glider. Note that they are starting at the edge of a beginning downslope. This means that — since they are facing a head-wind — that the wind is rushing up the slope, and therefore has a significant vertical component.

Hence, in relation to the moving air-mass, the bird and the glider are gliding down, but because of the air's upward motion — in relation to the ground — it can cancel out or — in the case of the glider — overcome the downward motion of the flyer.

After that it is merely a question of adapting horizontal drag such that the horizontal velocity component is also cancelled out, and that enables some birds to hover apparently motionless.

U_flow · Answer 4 · 2021-09-20T13:03:00.833

By very accurat direct variation of lift and drag of its wings.

As Nasa and the other answers point out, three main forces act upon the bird. Lift, drag and its body weight. The (controls) problem for the bird is now to balance these three forces, which is further complicated as it also has to keep its rotational attitude while the wind rapidly changes magnitude and direction.

The bird is only able to achieve this by directly modifying the lift and drag forces it is generating with its wing. You can actually see that on the video, the bird rotates its wing (in order to generate more lift) or it slight folds and unfolds its wings in order to vary its wingspan (in order to decrease lift and drag combined). The key is that it can directly affect how much lift and drag is generated. If were unable to do so, the bird would either rise up or down, or would get carried away horizontally in either direction. You can see this exact effect in your second edit, in which you reference a hang-glider, which climbs away.

As a side note: It also has to keep its rotation, which it achieves with its tailfeathers and differential lift components of its wing as well as differential drag of its wings. This goes to show that this is an increadible feat of the bird to control its lift and drag forces as well its rotation just such that its head is able to balance out the remaining motion. This is simply incredible.

P.S. perhaps this question would be a better fit for aviation.stackexchange...

I think the reason it's on physics, is that it would be a violation of conservation of energy for a glider to maintain altitude without an updraft, and the OP hadn't considered updrafts a requirement for this to work. — Rick, Sep 22 '21 at 13:58

score 1 · Answer 5 · answered Sep 19 '21 at 16:45

Basically, the lift force generated by the air passing around their wings is not perfectly upwards perpendicular to the ground; instead, it is a bit tilted forward. The forward component of that force happens to be exactly the same as the backward component of the wind, while the upward component of the lift happens to be the same as the gravity force. All the forces cancel out, and the bird is hovering motionless.

AnoE · Answer 6 · 2021-09-21T07:04:53.687

The bird is using a relatively simple closed control loop to maintain its position. As shown in the diagrams of the forces acting on airfoils/airplanes, there are forces in all directions, and their concrete value depends on the geometry of the wing(s). The animal, in turn, has techniques to move in all directions by relatively minor movements, and "just" cancels out the wind movement.

If you look at your videos you see that the birds, while not quite flapping, do in fact move their wings and tail substantially. This constantly updates their position and velocity relativ to whatever target they want to meet, as the "microspeeds" of the wind around the birds change.

You see something similar in, say, wildwater kajaks surfing in standing waves. They stand still on absolutely ferocious water with very little effort (all the paddling you see going on there are to perform their tricks). With a bit of practice, you can stand very still in a wave (if it is less turbulent than in this example and you're not that hyperactive as these persons) with only minute weight shifting and a very occasional little touch of the paddle on the water surface for braking. In this case, the forward force cancelling out the water movement comes through the angle of the boat and gravity.

The same is true for the bird, and quite visible in the videos. If the bird needs to move forward relative to the wind (but not to the floor), it just does whatever a bird does to fly forward (i.e., angle its wings a little bit "down").

The real magic, for me, is, how that little thumbsized brain (if it is that large at all) manages all that processing in realtime. Nature on fire!

There's actually a lot of sensor information there... Visual lock on the ground for position reference, accelerometers in their ears, many birds have at least a bit of a magnetic compass in their brain, and all those feathers are attached to nerves that can feel the air movement around them. If anything someone trying to duplicate that little brain would likely be overwhelmed by too much sensor information. — Perkins, Sep 20 '21 at 16:13
@Perkins: If bird brains amaze you, then you've never reallly thought about e.g. dragonflies :-) — jamesqf, Sep 20 '21 at 21:13
The neural density of avian brans is higher than it is in mammals. — JimmyJames, Sep 21 '21 at 17:45
If you settle for fruit flies instead of dragonflies, there are pretty complete brain maps. E.g. https://bit.ly/2GKmDF2 from https://ai.googleblog.com/2019/08/an-interactive-automated-3d.html. If only we could find the neuron that makes it hover! — Daniel Darabos, Sep 22 '21 at 09:41

How can a Kestrel hover in the wind?

6 Answers6

Only if there is an up-draft