The Why of Fly - The Origin and Evolution of Flight in Birds: Part 2

To read The Why of Fly - The Origin and Evolution of Flight in Birds: Part 1 by Gavin Leighton, Click here.

How flight evolved is one of the oldest and yet most uncertain questions in avian evolutionary morphology.  Below, Gavin explains some of the prominent hypotheses that may explain the evolution of flight in the context of ecology.  White Hawk, Chan Chich, Belize [Photo by Lukas Musher]

In the first post I explored some of the requisite physiological/morphological changes necessary for flight in birds.  Importantly, these physiological changes did not arise in a vacuum, and there remain the interesting questions of what selective forces could change the body plan of birds so that flight was possible.  Since flight evolved in birds millions of years ago, one can not definitively define the specific selective pressures that contributed to bird evolution.  Despite this difficulty, we still have a set of several competing hypotheses for the selective factors promoting flight.  These factors are described below in conjunction with the evidence for each hypothesis. 

The first explanation is the oldest explanation, having been proposed in 1879 by Samuel Williston.  This first explanation for flight is the cursorial hypothesis (Figure 1).  The cursorial hypothesis posits that the bipedal ancestors of modern birds would run to catch their prey.  To facilitate catching prey that was flying away (think insects), the ancestors would leap into the air to obtain the prey.  While the cursorial hypothesis seems technically possible, the theory is not parsimonious.  First, to gain sufficient ground speed for considerable ascent the ancestors of birds would have to have been faster runners than the birds today.  Second, after liftoff, the increased drag would after liftoff would have limited the ascent.  Finally, and perhaps convincingly, we don’t see this behavior in any extant birds today, suggesting that either this explanation is erroneous, or that feathers evolved according to the cursorial hypothesis and then the behavior was subsequently lost by any and all ancestral birds. 

Figure 1: The cursorial hypothesis.  Therapod dinosaurs that could achieve short bursts of lift may have been better able to catch flighted prey (i.e. insects; dinosaurs like Archeopteryx were not much larger than an American Robin), thus giving them an advantage in survival and reproduction.  Unfortunately this hypothesis is not well-supported.

One of the major hypotheses for why flight evolved capitalizes upon observations of contemporary birds.  Since many birds spend significant time in the trees, the arboreal hypothesis of flight argues that wings evolved to help birds navigate from tree to tree.  The progression of evolution begins with individuals living primarily arboreal lifestyles (i.e. foraging in trees and spending most of the time in tree canopies).  Such a lifestyle would put selection pressure on individuals to move from tree to tree without having to return to the ground first.  These observations led to the arboreal theory.

The arboreal theory is the most strongly supported theory and also provides a plausible progression of feathers.  Specifically, the first arboreal individuals would have utilized the feathers to glide from branch to branch, instead of flapping their wings.  Indeed, research indicates that many of the early feathers would not have been able to withstand the force of a downstroke during flight (Nudds and Dyke, 2010).  Since individuals could not flap their wings to take off, one would expect that the first flight, or proto-flight, took place when birds would jump from branches to reach another branch. 

The strength of the arboreal hypothesis derives from multiple sources.  The first is that the requisite physiology necessary for flight was not present in many feathered theropod dinosaurs, and thus, self-powered flight was not possible.  Therefore, climbing a tree to achieve flight would explain how flight could be achieved without all of the pieces being in place.  Second, contemporary birds are arboreal, and inhabitat almost every vertical niche one can think of.  Third, there are many other arboreal inhabitants that have evolved the ability to glide due to their arboreal lifestyle.  For example, flying squirrels and lizards with skin flaps jump from trees and use various adaptations to glide to another branch.  And finally, the arboreal hypothesis provides an argument for the extensive feathering we see on the bodies of Microraptor and Archaeopteryx (Figure 2). 

Figure 2: Many therapod dinosuars in the avian lineage, such as this Microraptor, are known to have been covered with  feathers,  including long feathers extending from both forelimbs and hindlimbs, as well as from the tail, suggesting that early birds were gliders rather than capable of powered flight.

An explanation on the periphery is that wings were primarily helpful for young birds that would climb trees to return to nests they had fallen from.  This idea, known as assisted-incline running is argued most forcefully by Ken Dial (Dial, 2003).  Dr. Dial has studied chukars (Alectoris chukar) in the lab and notice that they will pump their wings to scale inclines in the lab (Figure 3).  The fact that there is a modern bird that uses wing-inclined running makes it more attractive than the cursorial theory that is not supported among modern birds.  In contrast, the theory suffers from fossils that are incongruent with wing-assisted incline running.  Specifically, fossils such as microraptor have feathers on both the hindlimbs and tail; and the feathers in these areas would be unnecessary if used for wing-assisted incline running.   

Figure 3: One potential explanation for how flight evolved involves using wings to help scale inclines.  Although plausable and supported empirically, it probably isn't as good of an explanation as the tree-down, or arboreal hypothesis. 

Most recently, a group from Montana State University has proposed that the evolution of feathers in theropod dinosaurs was used primarily to help stabilize the predator while it was pinning it’s prey with feet (Fowler et al., 2011).  The argument is that theropod dinosaurs, like birds of prey today, would pin their prey down using both feet.  Pinning the prey was enhanced by strong legs and large talons that are used to hold prey that are large enough that they may escape.  Importantly, once the prey has been pinned, the prey may still struggle, thus causing the theropod to lose balance – since it’s legs are being used to grasp the prey.  To help stabilize the predator, the authors argue that feathers would have evolved and wing beats could be used to stabilize the predator while it consumed the prey.  Similar to the other hypotheses, this idea is plausible; however, it still does not explain the extent of the feathers on the entire bodies of many of the earliest bird ancestors. 

Similar to the diversity of birds we see today, there is a diversity of hypotheses that have been offered to explain the evolution of flight in birds.  The four hypotheses: the cursorial, arboreal, wing-assisted inclined running, and predator stabilization, all provide potential explanations for flight.  Some of these hypotheses are even reinforcing.  For example, an arboreal lifestyle would have likely favored making nests in trees, which would have then favored individuals that fell out of nests to re-ascend into the tree.  Therefore, the non-mutually exclusive arboreal and wing-assisted incline running hypotheses could complement each other.  In total, however, the main hypothesis that is still considered the most likely is the arboreal hypothesis.  The arboreal hypothesis can explain many of the phenomena we see in extant birds, and much of the physiology in ancestral birds.  Therefore, birds arguably evolved flight to glide first, and over time gained the adaptations necessary to perform powered flight.  Thus resulting in the avifauna we see today. 

Barrow's Goldeneye, Rodeo Lagoon, Marin Headlands, CA [Photo by Lukas Musher]

By Gavin Leighton


Gavin is a PhD candidate at the University of Miami studying cooperative behavior in Sociable Weavers.  To learn more about Gavin, see our Guest Writers page.

Citations:

Dial, K. (2003). Wing-Assisted Running and the Evolution of Flight.  Science. 17: 402-404

Fowler et al. (2011) The Predatory Ecology of Deinonychus and the Origin of Flapping in Birds. PLoS ONE 6(12).

Nudds, RL., Dyke, GJ. (2010). Narrow Primary Feather Rachises in Confuciusornis and Archaeopteryx Suggest Poor Flight Ability.  Science. 14: 887-889. 

The Why of Fly - The Origin and Evolution of Flight in Birds: Part 1

By Gavin Leighton

The most salient trait of birds is their ability to fly. Flight allows peregrine falcons to dive at just over 200 miles/hour, flight allows for migration, and flight allows birds like albatrosses to make vast expanses of ocean their home. Indeed, the evolution of flight was a critical event (and maybe the critical event) in the evolution of birds. Given the importance of flight for the class Aves, evolutionary biologists have long studied the origins of flight, and researchers are beginning to delineate how and why birds evolved flight.

Peregrine Falcon, Point Reyes, CA.  Dec. 2011 [Photo by Lukas Musher]

How birds evolved wings with flight feathers is an interesting story, especially since flight has evolved independently at least four times (insects, bats, pterosaurs, and birds). To begin I will very briefly outline the major physiological steps that produced wings that could produce self-powered flight. In the second part of the series, I will try to describe the selective mechanisms that favored the following physiological changes.

Birds’ closest ancestors were theropod dinosaurs (1). We all know what theropods dinosaurs are thanks to Jurassic Park (and as far as I’m concerned, only one Jurassic Park movie was ever made and it came out in 1993). As depicted in Jurassic park dinosaurs such as Velociraptor (see below), which is actually much smaller than depicted in the movie , and Tyrannosaurus, which may have been more of a scavenger than a predator, are both examples of theropod dinosaurs. In general, theropods were bipedal dinosaurs with strong hindlimbs and a long tail that was likely used for balance. From theropods we get birds, here is a woefully brief description of how.

An artist's rendition of velociraptor [Wikipedia]

To get from velociraptor to a contemporary bird, the first thing we need is feathers. Feathers were traditionally thought to only occur in birds, but a cascade of recent evidence demonstrates that many theropod dinosaurs had feathers as well (2) – I know what you are thinking, and no, Tyrannosaurus Rex could not fly. The theropod feathers were not used for flight, so other explanations must be invoked. The two common explanations are that the dinosaur “protofeathers” were used for signaling during sexual selection (due to the types of melanin researchers have found in fossils we know the dinosaur feathers were black, white, or red-brown type colors [3]); or, for facilitated thermoregulation. These two ideas are not mutually exclusive and hopefully future work will indicate the selective pressures that favored the evolution of the first feathers.

An artist's rendition of anchiornis [National Geographic]

At this point we have feathers on a roughly bi-pedal animal, but we still aren’t close to powered flight. To get powered flight we must move from the proto-feathers (see below) here is a very simple model of how feathers evolved) that resemble the downy feathers of chicks, to asymmetrical flight feathers. To get to contemporary flight feathers, feathers passed through intermediate stages, and an excellent review of this process was “Which came first, the feather or the bird?” by Prum and Brush (2003 – see citation 2). One of the main intermediate stages is the symmetrical flight feather (3). The symmetrical flight feather, compared to the asymmetrical flight feather, is actually worse for flying due to physics that will not be described here. Despite problems with a symmetric feather, components of the symmetrical flight feather were useful. For example, there are tiny hooks called barbules that help smooth the feather to maintain a feather’s airfoil shape. The evolution of barbules almost certainly occurred with or after the evolution of the symmetrical flight feather, thus making the symmetrical flight feather a useful stage to focus on. So, let’s skip ahead a bit and assume we have some sort of flight feathers on the arms, what comes next is the development of adequate wings.

Proto-feather evolution in chronological order. [Wikipedia]

The change from forelimbs with separated digits to the common bird wing involved numerous skeletal changes that I won’t describe.

Wings, such as the long wings allowing Black-footed Albatross to wander countless miles over vast expanses of ocean
evolved from the forelimbs of therapod dinosaurs. Pelagic out of Fort Bragg, CA. Aug. 2011. [Photo by Lukas Musher]

What I will mention as an aside is the downstream evolutionary consequences after wings evolved. After proto-birds lost any sort of forelimb dexterity, the duty of manipulating objects was transferred to the beak. This transfer likely explains the extraordinary diversity of bill forms, compared to other groups. So lets assume that the wing morphology is coming into place, even with wings, other physiological structures and adaptations were necessary to yield contemporary birds. Among the adaptations are hollow bone structures that largely reduce the weight of a bird (1), and save the bird significant amounts of energy during flight. And to actually get into flight, birds rely on their oversized breast muscles, which became so powerful that they need to be anchored to a keeled sternum.

Archeopteryx lithographica, Wyoming Dinosaur Center.  Taxonomically the first bird.  One of 8 specimens worldwide.
Note the details of feathers and other traits that link dinosaurs to birds including the furcula. [Photo by Lukas Musher]

All of these physiological adaptations compose the part of the story that describes “how” bird physiology changed over time to allow for powered flight. In addition to how birds anatomy changed, a complementary question is “why” birds are the way they are, and not otherwise. To answer why birds can fly, several ideas have been posited with varying degrees of support. In part 2 of this series, I will limn the most well established ideas for why birds evolved flight and some of the newer ideas as well. 

Gavin Leighton is a PhD candidate at the University of Miami, studying cooperative behavior in Socialble Weavers.  See more on Gavin at Guest Writers.

Citations:
1 Kaiser, G. The inner bird: anatomy and evolution. 386 (2007).
2 Prum, R. & Brush, A. Which came first, the feather or the bird? Sci Am 288, 84-93 (2003).
3 Vinther, J., Briggs, D. E. G., Clarke, J., Mayr, G. & Prum, R. O. Structural coloration in a fossil feather. Biol Letters 6, 128-131, doi:10.1098/rsbl.2009.0524 (2010).

To read the Why of Fly - the Origin and Evolution of Flight in Birds: Part 2, click here.