Analyzing a Trade-Off, Swiftly

Nick Minor
Jun 20, 2015 · 6 min read

(Originally published March 26th, 2014 on

Common Swift — Image Credit: Wikipedia

If you’re a researcher looking to study flight, birds are a pretty good place to start. Birds have achieved flight speeds from 200 miles per hour all the way down to 13, with everything in between. Birds have flown extended journeys well above Mt. Everest and well below the surface of the ocean, mastering flight both in the jet stream and in the most powerful of ocean currents. Certain birds have adapted to completing sustained, unbroken migrations across more than 7,000 miles of the Pacific Ocean in only nine days. Migratory birds in North America that weigh less than an ounce take on flights straight across the Gulf of Mexico twice every year. Some birds fly by barely flapping their wings at all, using thermals to soar across hundreds of miles of mountain range per day. Others flap their wings more than 80 times per second for the hovering precision needed to feed constantly on nectar. Some birds can chase fleeing forest birds through dense tangles of vegetation at 30 miles per hour, while others can pursue duck prey over arctic tundra with sustained flight speeds of 70 miles per hour. Others still can carry loads of fish that weigh up to half the body weight of the avian carrier.

But perhaps one of the best places to start, amongst the overwhelming variety and splendor of bird’s feats in flight, is with the birds that do it the most. Swifts — a family termed Apodidae, derived from the Ancient Greek work for “without feet” — hold the record for spending the highest proportion of their lives on the wing. The record-holder for the longest sustained flight is the Common Swift of Eurasia, ranging between 2 to 4 years of nonstop flight at certain ages.

Swifts provide a frontier to press towards in the understanding of avian flight. Being capable of such sustained flight, however, does not mean being “ideal for flight” in general, though of any bird, swifts would be fairly close to that ideal. Platonism does not work in nature, and we might as well wash that from our brains now. Everything that makes any animal what it is exists purely based on the pre-existing genetic material and how that material adapts to the conditions it finds itself in. Swifts are not ideal for flight. They are ideal for flight in “x” circumstances, or in “x” conditions.

Being able to sustain flight for years means that swifts are able to adapt to an enormous array of conditions aloft, some of which we, as land-bound primates, can’t understand. Swifts must adapt to random wind shifts from calm to extreme speeds, shifts in wind direction, changes between rising and falling columns of air, differences between air above water and air above land, rain, hail, snow, sleet, fog, blinding sunshine, and they have to do so while navigating the skies and finding enough food to sustain such an effort.

But fear not…swifts have a slurry of adaptations to make them such masters of the sky. First, swifts have a body shape to reduce what’s called parasitic drag — the drag that is inherent to any body trying to sustain lift in the air. This also means having tiny feet that don’t get in the way of airflow. This lends itself to an issue: swifts are incapable of perching, but their feet have adapted to the next best thing. Because swifts can only cling to vertical surfaces of crawl along flat surfaces, they have adapted a unique and muscular toe configuration called pamprodactyly. Swifts are also capable of what’s called unihemispheric slow-wave sleep, which means they can sleep with one side (hemisphere) of the brain at a time, leaving the other half awake to control flight and navigation. Behaviorally, swifts collect all of their nesting material from debris in the air, drink in flight, court, and even mate in midair.

Only the sections here labeled with X contain
the swift’s arms. The rest of the wings are
feathers, without any bones. Image from Henningsson et al. 2008

Perhaps the most important adaptations swifts boast is their wings, and even more importantly, how they use them. Swifts have a fairly unique physiology of the wing: the actual arm portion of the wing is extremely short, with the wrist of the wing placed quite close to the body. Most of the wing, then, is the outer wing feathers that go from the wrist and form the tip of the wing: the primaries. In swifts, the bones that control the wingtip are also relatively large. Why? With these relatively small modifications to typical avian wing morphology, swifts have enormous control over the angle and shape of their wings, which allows them to adapt to changing conditions aloft and retain control and maneuverability. Also impressive, swifts share the ability to rotate their wings at the base with hummingbirds. This allows their wings to be fully extended — and thus to generate the most lift — on both the upstroke and the downstroke. With this sort of complexity already present in swifts, a recent study reveals one of my favorite evolutionary phenomena in swifts: a trade-off.

Swifts are renowned both for their gliding flight and their flapping flight, but both require very different physiological adaptations — contrast the wings of predominantly gliding condors with those of predominantly flapping ducks. One would expect, when looking at swifts, that they are equally well adapted to flapping flight as they are to gliding flight. Makes sense right? This way, swifts would be ideal for flapping or gliding.

But remember, there are always more conditions, more variables, to consider. In this case, we know that swifts benefit from the highest possible level of efficiency, and ultimately, this means being efficient with energy. So let’s ask the important question. Which kind of flight requires the most energy: flapping, or gliding? The answer to us seems obvious. While gliding is relatively passive, flapping requires constant effort of a complex muscle system and takes much more energy. This discrepancy is where we find the trade-off — in order to maximize efficiency in flight overall, swifts must balance their efficiency at gliding with their efficiency at flapping, because, after all, they can’t switch between a body optimized for flapping and a body optimized for gliding every time they switch flight styles!

This is figure 1 from the first study cited below. Dark blue corresponds to positive lift, or upward motion, which, for our purposes, means best possible efficiency.

And like with all trade-offs, evolution handles this one beautifully. Because swifts spend much more energy at flapping, swift wings have adapted more towards efficiency at flapping to minimize this energy cost. They are “flapping-biased”. While they may be less efficient at gliding because of this, gliding required less energy to begin with. By minimizing the energy cost of flapping rather than adapting equally to flapping and gliding, the energy cost of flying overall is less for the swifts. It’s a balancing act folks, and in this one, leaning toward flapping works best. Isn’t that fascinating?!

The level of complexity in animals we can sometimes take for granted, like twittering swifts spiraling overhead, is dumbfounding; we have so much to understand even just outside our bedroom windows. If we could only see the gray of the unknown in the world around us, like some strange, brain-wave-reading Google Glass app, we would see an inordinately gray world.

And in order to clear some of the gray with swifts, the scientists in the above-mentioned study only needed a curious eye, some math, and a stopwatch. Oh and a windtunnel. Don’t forget the windtunnel.

Have a great day everybody.


Henningsson, Per, Anders Hedenström, and Richard J. Bomphrey. “Efficiency of Lift Production in Flapping and Gliding Flight of Swifts.” PLOS ONE. PLOS ONE, 28 Feb. 2014. Web. 26 Mar. 2014 <>.

Henningsson, P., G. R. Spedding, and A. Hedenström. “Vortex Wake and Flight Kinematics of a Swift in Cruising Flight in a Wind Tunnel.” The Journal of Experimental Biology. The Journal of Experimental Biology, 2 Jan. 2008. Web. 26 Mar. 2014. <>.

Stories about biodiversity & why earth has so much of it

Nick Minor

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Aspiring explorer-naturalist with a passion for birds, big questions about biodiversity, storytelling, & adventure.

Stories about biodiversity & why earth has so much of it