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Are there advantages to the way some birds' wingtips end in separate feathers instead of a more solid shape?

As an example, separate-feathered wingtips:

enter image description here

more solid wingtip: enter image description here

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  • $\begingroup$ Absolutely there is an aerodynamic advantage and it's not really about " feeling" the air. No time to compose a real answer right now. Not to say that that solution is "better" than albatross but it has some specific points in its favor. It has been tried on some airplanes-- look for related questions on ASE. More on some other day. $\endgroup$ – quiet flyer Nov 13 at 15:39
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    $\begingroup$ Notice that, besides swallows, seafowl have solid wingtips. The feathered wingtips are found on birds that must fly through branches of trees. They stretch the wing with flexible material that doesn't get damaged when the bird misjudges the available space - similar to a cat's whiskers. The ocean surface and cliffs have no trees, so swallows and seafowl can afford to have the more efficient solid tips. $\endgroup$ – Peter Kämpf Nov 13 at 20:54
  • $\begingroup$ Continuing on Peter's comment, one can notice that a) seabirds have pointed wingtips not slotted wingtips, and 2) seabirds never have dihedral, and often have anhedral on the outboard portion of the wings, which dominates the balance of roll torque. The former point (a) is well suited to high flight speeds and the latter point (b) is well suited to high winds. Heavy wing loading/ high flight speeds also are good adaptations for a windy environment. $\endgroup$ – quiet flyer Nov 14 at 0:21
  • $\begingroup$ I'll incorporate all this into a real answer in the future! Anyway, generally speaking, the birds with slotted wingtips are not the ones that fly at high airspeed or more accurately a high "scale airspeed". $\endgroup$ – quiet flyer Nov 14 at 0:23
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    $\begingroup$ But I would also add that while accipiters fly through the branches of trees, Red-tailed hawks, Turkey vultures, and California condors do not. Neither does the vulture in the top pboto in the question. So that's not the key variable re slotted tips. More later. $\endgroup$ – quiet flyer Nov 14 at 0:25
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Slotted wingtips provide torsional flexibility.

A bird’s wingtip feathers must twist in one direction during the upstroke of the wings and in the other direction during the downstroke to keep the local wind striking the wing at an appropriate angle to generate lift and thrust... The turning could be done at the base, with a completely inflexible feather; the aerodynamics are improved and material saved if the local flow forces twist the feather by just the right amount.

-- S. Vogel, Comparative biomechanics: life's physical world, p. 382, 2003.

They also reduce drag.

The minimum drag of a Harris' hawk gliding freely in a wind tunnel was measured before and after removing the slots by clipping the tip feathers. ... the feathers that form the slotted tips reduce induced drag by acting as winglets that make the wings non-planar and spread vorticity both horizontally and vertically.

-- V. Tucker, Drag reduction by wing tip slots in a gliding Harris' hawk, Parabuteo unicinctus, J Exp Biol 198:775-81, 1995.

They are effective while flapping, not just while gliding.

We used particle image velocimetry to measure the airflow around the slotted wing tip of a jackdaw (Corvus monedula) as well as in its wake during unrestrained flight in a wind tunnel. The separated primary feathers produce individual wakes, confirming a multi-slotted function, in both gliding and flapping flight.

-- KleinHeerenbrink et al, Multi-cored vortices support function of slotted wing tips of birds in gliding and flapping flight, J R Soc Interface 14(130):20170099, 2017.

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They function like an array of winglets. Each feather is aligned to optimize its angle of attack in the local flow, which is circulating from the bottom to the top around the tip, to extract energy from the circulating flow, weakening the circulation (by redirecting it the other way - a wing deflects air to make lift) and providing a beneficial lift/thrust component ("thrust" to the extent that the airfoil is angled nose down to achieve an optimal AOA, and lift being 90 deg to AOA).

The result is a wing with very low aspect ratio that regains a bit of the efficiency lost with a low aspect ratio wing, where the benefits of low aspect ratio are important for the bird (maneuverability and ability to fight). I'm sure that these benefits are magnified at the very low Reynolds Numbers that bird wings operate at (to the bird, the air is way more viscous than to the airliner, and to the bee, the air is like motor oil).

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    $\begingroup$ Another factor here is that it's much more efficient (lighter, better control, &c) to implement this array of winglets with muscles and nerves, rather than a bunch of electric or hydraulic servos. $\endgroup$ – jamesqf Nov 13 at 17:43
  • $\begingroup$ Yep. I'm not religious at all, but the way everything in nature is so precisely "engineered" just leaves me in wonder at the mystery of it all, and very unsatisfied with Darwinist theory. There is just.. more... somehow. Dunno what. $\endgroup$ – John K Nov 13 at 17:53
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    $\begingroup$ Maybe a little mathematics will help @John K : using Stetson-Harrison method, we estimate it took ten million years for the different wingtips the contemporary birds to evolve (perhaps under the supervision of the allmighty, who knows), and again, with S-H, we estimate the male and female of the species to procreate at the age of three. So, to this date, there has been a total of 3 333 333 generations of birds since the common ancestor to evolve. It doesn't take that much mutation per generation to produce such a wide variety of our featherry friends. $\endgroup$ – Jpe61 Nov 13 at 19:29
  • $\begingroup$ I'm all in with genetic mutation and evolution and all that, to a point. Still doesn't explain everything. For example, DNA is a code. A set of instructions. A language, really. Is that random? There are still questions that science can't answer for sure. If there was a big bang, what was before that? And before that? I just revel in the wonderful mystery of it all. $\endgroup$ – John K Nov 13 at 20:16
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    $\begingroup$ @JohnK: there is an abundance of stuff in the natural world that is a long way from "precisely engineered", it's hardly "everything". $\endgroup$ – whatsisname Nov 14 at 0:43
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Birds with wingtips like this tend to be thermal soarers, so they probably help the bird detect air currents. When circling in a thermal, you need to know where the air is rising most strongly so you can avoid flying out of the lift. Glider pilots feel one wing rising or falling, but a bird with delicate feathers could feel the differences between the actual airflow at each tip and respond faster.

The swift in your second photo is not a soarer. It requires agility to catch insects mid-air.

Some birds (like an Albatross) are extremely efficient gliders with a solid wing tip. They glide on much faster moving air that must be easier to feel, so they're shaped for efficiency rather than sensing.

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  • $\begingroup$ See comment under question-- $\endgroup$ – quiet flyer Nov 13 at 15:42
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    $\begingroup$ I've shared a thermal with a soaring bird. Watching their wingtip feathers respond to very small variations in air currents was one of the neatest things I've ever seen. It seems as if each feather is adjusted individually to the air hitting just that feather. $\endgroup$ – Wayne Conrad Nov 13 at 19:28
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    $\begingroup$ @FreeMan - thanks, I've fixed it. $\endgroup$ – Robin Bennett Nov 14 at 9:03
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    $\begingroup$ @Wayne each feather does adjust to the local airflow, but as a side effect of aeroelasticity. There isn't one muscle per feather! (Actually this robust passive design is more impressive than one with dozens of sensors and actuators.) $\endgroup$ – Camille Goudeseune Nov 14 at 15:45

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