Evolution of Flight: Did Early Birds Run and Flap Before They Flew?

By Veronique Greenwood | June 24, 2011 1:03 pm

Flapping while running up a ramp takes far
less energy than flight at the same angle.

What’s the News: How did birds get their wings? And how did they start using them to fly? These questions have bedeviled evolutionary biologists for more than a century, and with flight’s origins long buried, a lot of careful measurements of how modern birds work combined with clever guesswork has resulted in several fiercely differing theories. The two major camps have proto-birds either dropping from trees or running along the ground before finally taking to the air.

A new study lends credence to the idea that flapping wings while running could have been involved by showing that it requires much less energy than flying while still helping birds get over obstacles. This suggests that it could have been an easy way for proto-birds to start going through the motions.

How the Heck:

  • The researchers had noticed that young birds running up ramps and other obstacles flap their wings strongly, gaining speed and balance. The team wondered how much energy the process took: as the behavior gets birds over obstacles as effectively as actual flight, if it took less investment of energy, it could have been a flight stepping stone accessible to early birds.
  • They trained adult pigeons to run up nearly vertical ramps as well as fly to perches from the ground, requiring a similar angle of ascent, and then implanted sensors in their flight muscles. When they recorded how much energy each took, they found that running took less than 10% of the energy of flight. This confirmed that flapping while running up obstacles could have been a plausible intermediate stage in flight’s evolution, and could have been performed even with fairly small muscles, such as those in early birds.

What’s the Context:

  • The two primary competing theories, arboreal (tree-dropping) and cursorial (running along the ground), imply totally different lifestyles for early birds. Thus, a lot of the debate over these models, which were first suggested in the 1880s, revolves around what bird-like fossils like the feathered Archaeopteryx reveal in terms of tree-climbing or running ability. But interpretations of the structure of its bones, claws, feathers, and probable musculature differ considerably.
  • The evolution of feathers has likewise been drawn into the fray, as when they arose, and which ones, pinions or secondaries, came first, matters in models of early flight (see here for a detailed discussion of both models and the evolution of feathers).
  • Ken Dial, the senior author of the new study, came up with the idea that studying how young birds learn to fly could give scientists insight into how flight evolved in the first place. While there’s a whiff of recapitulation theory there (the now-discounted idea that a creature’s development recapitulates its evolution), in terms of physics, the pre-flight motions of young birds are one of the few ways scientists can get a look at what might have happened at a point when creatures had wings but did not yet fly.
  • Though the involvement of running seems to place this work in the cursorial camp, Dial, citing problems with both the major models, has said that he sees his work striking off in another direction. “I would argue that studying the proto-wings of juvenile, still-developing birds might provide a useful avenue by which to approach the problem of the earliest flyers,” he says (via CABINET). “Rather than concentrating on the arboreal/cursorial divide, I think it might be more fruitful to focus on questions like these, such as how does the flight stroke itself develop, what structures make it possible, and what purposes can they serve at differing levels of efficiency.”

The Future Holds: As long as time travel is impossible, studying the physics of modern birds and comparing them with fossils is the most effective way to learn about the evolution of flight. Expect more experiments dealing with the mechanics of flight, as researchers focus more and more on detailed aerodynamics in sussing out flight’s origins.

Reference: Jackson, B. E., Tobalske, B. W. and Dial, K. P. (2011) The broad range of contractile behaviour of the avian pectoralis: functional and evolutionary implications. J. Exp. Biol. 214, 2354-2361.

Image credit: Dial, K.P., J. Exp. Biol.

  • amphiox

    The study author makes a good point about how this finding should not be viewed as supporting the cursorial over the arboreal theories. As among the natural inclines that this behavior could help a bird run up would be tree trunks and branches.

  • orang

    Not exactly on topic, but I always found it a compelling hypothesis that some of the early wing and wing size evolution could have been driven by temperature regulation concerns or other protection from the elements [not just feathers, but the larger wing structure, too].

    Once large wings were present, other theories come more into play. If selection pressure is strong enough to keep large wings for other reasons, it would then beg to have the wings also help balance, jump higher, propel the bird forward when running, etc. These would all lead to components of the eventual motions of flight….

  • scribbler

    Uh, why not just study chickens??? They have wings and don’t fly…

  • Glidingpig

    Chickens can fly, just not far.

  • m

    i might have started this study to look at swimming as well. wings may have first developed as paddles for pro-birds to catch fish.

    flight may have been inveitable as these bird like things lived near sources of food….high cliffs, etc.

    over time…say contintental drift, as a lot of coastlines “rose” higher, evelotuion may have followed.

  • atlanticblue

    I suspect they used their wings to escape predators faster, on cliffs perhaps. I take care of parakeets whose wings are cut and they are hard to catch when they boost themselves further with their wings. I always catch them when they are running. Also, being hit by a wing in the face hurts. A dinosaur bird slap in the face must have stunned even more.

  • http://www.nicky510.com Crow

    This avenue supplies a very reasonable avenue of “small steps.” Flap harder, or have longer feathers, and you can maneuver just a bit better. Escape just a bit more and pass on the genes. Seems like a very small step from heavy flap/run to flap/fly.

  • A. Reader

    I do not like to use e-mail, for various reasons. May I suggest that you put a “feedback” link to a text-box such as this one on your main pages, so that someone may give feedback quickly and easily?

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    Your readers perhaps could contribute to improving the quality of your publication (which already is of more than acceptable quality!), even in such mundane areas as proofreading, Facilitating their feedback would encourage this.
    Perhaps the text feedback could be limited to perhaps 100 to 200 characters, to discourage spam and rants.

    Should another reader wish to e-mail the publication with this suggestion, it would be much appreciated. Thank you.


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