Flight in Birds

Flight is the main method of travel and the main characteristic describing Birds,Class Aves. It is a large evolutionary adaptation providing birds many advantages and benefits. Flight has opened the Aves to many different niches throughout the world and large species diversity is the product.

Flight has included many evolutionary steps, involving certain behaviours, mechanisms, adaptations and even in some cases a loss of that particular skill. Flight has evolved as a main form of locomotion in several other lineages of animals as well.

Pigeons caught showing different phases of flight

Behavioural Mechanisms to Arrive at Flight

There are a number of ideas surrounding the origins of powered avian flight. In order for powered flight to occur some significant body plan changes were required. For example, feathers evolved before flight but became secondarily useful . Proto-birds also gained features such as hollow bones, a highly specialized excretory system, an enlarged cerebellum as well as modifications to their skeletal structure and respiratory system. The oldest known living ancestor to modern birds is Archaeopteryx, who lived up to 150 million years ago (Hedenström 417) in the Jurassic Period.

Archaeopteryx

The Arboreal Model

The Arboreal Model is a trees down model, which focuses on the idea that animals arriving at flight started by climbing trees, rocks or cliffs and began to jump, managing to glide to other locations. This mechanism was first proposed by Darwin in 1859 for bats; however it was Marsh in 1880 who proposed this method to be the origin of bird flight as well (Norberg 303). There is evidence that suggests early ancestors of birds were in fact capable of climbing trees; even some modern birds have claws used for climbing (Norberg 305). The arboreal model also argues that a trees down method is the least energetically expensive way to begin flying. If an animal is able to take advantage of the food resources available in one tree and glide to the next then a great amount of energy can be conserved compared to other models. Another piece of evidence which supports the trees down theory is the presence of a dorsoventrally flattened tail (Norberg 306) in early ancestors of birds such as Archaeopteryx. This adaptation was critical for stability while gliding. Another early adaptation that would have been beneficial to gliders is the presence of symmetrically lengthened front limbs. In addition, it would seem that flapping could have come easily once gliding had been achieved. It is possible that gliders were also capable of pronating and supinating their forelimbs in order to achieve maximized control (Norberg 322-323).

The Cursorial Model

Also known as the ground up theory, the cursorial theory is the idea that birds arrived at flight by running and jumping, which led to powered flight. This model excludes the intermediate step of gliding. Current phylogeny favours this theory, as fossil evidence tells us that birds evolved from feathered cursorial dinosaurs (Hedenström 416). Some problems with this model include the idea that flight would have been evolving against gravity, not an easy feat; as well as the fact that leaping could have slowed them down. Supporters of the cursorial model have shown that taking off from the ground up may have been a possibility for Archaeopteryx . Initial analyses of the possibility suggested that Archaeopteryx would not have been able to run fast enough to achieve take off; he would have needed to reach a speed of 6 meters per second but his estimated top speed was just 2 meters per second. More recent studies on the other hand proposed that if Archaeopteryx began flapping his wings while running, this would have created enough lift to counter his proposed weight, allowing him to increase his speed to about 7.8 meters per second, fast enough to become airborne (Hedenström 417-418).

Pouncing Proavis

In 1999, Garner, Taylor and Thomas came up with the Pouncing Proavis Model; the idea that powered flight began with a combination of gliding and leaping. Contrary to the arboreal model, pouncing proavis began gliding for the purpose of attacking prey from heights with enhanced stability and control . Developers and supporters of Pouncing Proavis argue that it is the only model in which early flight adaptations can be correctly correlated to the current cladistics of theropod dinosaurs, which includes modern birds . Pouncing proavis is suggested to be a predator, whose pounce evolved into a swoop with selection pressures favouring ever increasing stability and control of the attack. Eventually as swoops became larger, flight as we know it today evolved (Garner et al. 1262) .

Flight Mechanics

In general, the means by which birds navigate their physical surroundings is through wing-powered flight. This section will detail some of the adaptations birds have acquired to help them to become proficient fliers, as well as the basic physical aspects of the act of flight. A large part of birds being proficient fliers, and having a large variety in flight capabilities, is due to the variety of different types of wings found in birds.

Wings

There are many different varieties of bird wings. One of the more important aspects of differentiating wing varieties is the aspect ratio of a wing, which is the comparison of how long a wing is to how narrow/wide it is. Aspect Ratio doesn’t directly correlate to any specifics, a higher aspect ratio doesn’t mean a faster flier, but it is useful for differentiating the different types of wings.

Elliptical Wings

Elliptical wings are short, broad wings, and have relatively low aspect ratio. They are common among smaller birds that require maneuverability in dense foliage while flying.

High Aspect Ratio Wings

Birds with longer, pointed wings are generally adept at slower flight, with lots of soaring relying on wind currents. The Albatross is a good example of a slower flier which is quite adept at soaring.

High Speed Wings

Shorter, tapered wings with a very high aspect ratio give birds very fast flight with minimal drag, these wings can be found in most falcons.

High-Lift Wings

High lift wings are longish, broad wings found primarily in larger birds. The wings have spaced feathers at the tip that, at slower speeds, help to maintain velocity without losing too much to wind resistance. These wings are typical of birds like Eagles, and Storks.

Hovering

Hovering requires a huge amount of energy for birds. This is the primary reason behind why larger birds cannot truly hover. Birds that achieve a true hover have to flap their wings 50-80 times a minute. Because of the large amount of energy required for hovering, it’s generally smaller birds that are able to achieve hover, more specifically: Hummingbirds. Humming birds are able to achieve hovering flight because of their rigid wings, and shoulder joints with a 180° rotation. These features allow them to have lift power on both the upstroke and the down stroke, and to pivot their wings forwards and backwards. Other birds can hover, but usually only for a brief period and not nearly as efficiently as the Hummingbird.

Basic Mechanics

There are four basic mechanics to bird flight they’re listed as follows:

Lift

In order for birds to fly, they must have lift. When the wings of a bird are pointed at an upwards angle, it creates a lower pressure in the air above the wing, causing the bird to rise or maintain it's elevation.

Drag

Drag is the encompassing term for the different frictions acting against the bird in flight. In order for birds to fly efficiently the have evolved to have sleeker bodies and light weights in order to counteract drag.

Gliding

Gliding occurs when a bird angles it’s flight slightly downwards. This allows the bird to produce enough forward force to counteract drag, allowing the bird to do little to no work and still move forwards.

Flapping

Flapping flight is how birds achieve flight. Birds use flapping to generate lift, the birds wings twist and fold while flapping up, and then spread out on the down stroke.. Twisting and folding help the bird to lift it’s wings without creating down force.

Half-wing traits pertaining to evolution of flight

Chukar Partridge

The origin of avian flight has been subject to much analysis and debate for quite a long time. Half wings were thought to be exaptations of flight. Due to new research of modern birds that most resemble ancestral proto-birds, this is no longer the case. It has been found that flapping wings, even half developed wings, helped ancestor birds run up steep slopes or climb trees by way of a constant fundamental wing angle. Behaviour similar to wing assisted incline running, flap running, or controlled descent is thought to have been used by feathered dinosaurs and other proto-birds, so that there is no great change in behaviour, just a gradual evolution to modern flight of birds.

Professor Kenneth Dial, of the University of Montana, was the first to discover WAIR during his study of juvenile Chukar partridges. He noticed that as they ran up ramps or flew to platforms, they never changed the wing angle formed during these movements. Chukar partridges were used for the study of WAIR as they are ground-dwellers who use bipedal running and incipient wings after hatching and inhabit complex environments, from cliffs to heavily forested areas. This species is a prime candidate to study as they best represented ancestral proto-birds to research the evolution of flight. This is still a relatively unknown process and much more research is required to fully understand this process and if in fact it is a realistic theory of flight evolution.

Wing-Assisted Incline Running(WAIR)

Wing Assisted Incline Running (WAIR) is a newly researched behaviour of mostly juvenile birds using flapping movements to run up steep inclines, such as cliffs or tall trees. During this vulnerable time of life, both young and old birds alike used this technique as a way of predation avoidance. By flapping their wings, aerodynamic forces push the body of the bird towards the substrate, whatever they are trying to climb, to give better traction as they make their way up. Due to this discovery, it is thought that proto-birds and ancestral birds, such as Archaeopteryx, used this technique since fossil records show that their wings were capable of making the right amount of force for this to occur. Through this discovery, it is now thought that this behaviour is a stepping stone for the evolution of flight.

WAIR has been observed in Galliforms, heavy-bodied ground-feeding birds also known as game fowl or gamebirds, as they are quite vulnerable to predation and known to live in environments with steep inclines, such as cliffs, rocky areas, and trees. This observation has been unique and quite functional by providing flight-incapable juveniles with access to more of their environments and the ability to reach elevated refuges. But not just juveniles use this behaviour. Even adults prefer to use WAIR over flight, exhausted birds incapable of flight are still able to use this behaviour, and birds with compromised wings can still use WAIR (Dial, Randall and Dial, 2006). It was found that this flapping behaviour did not require their wings to support body weight and does not take much energy to engage in WAIR due to the muscle tissue make up of wings and hindlimbs. Avian forelimbs are made up of fast-glycolytic fatigable fibers which require much energy to use effectively, while hindlimbs are oxidative non-fatigable muscle fibers which can still be used readily when the bird is physically exhausted (Dial, Randall and Dial,2006).

According to Dial, the mechanism of WAIR states: “developing ground birds employ their incipient wings adorned with symmetrical feathers to execute brief bouts of aerial flight, using dorsoventral flapping, and enhance hindlimb traction through anteroposterior flapping as they negotiate three-dimensional terrestrial environments.” (Dial, Randall and Dial,2006)

Mechanism and Aerodynamics

WAIR is, above all, a strategy for quick movement to elevated areas within an environment. This is done by flapping wings, in a similar way to flying, to push birds up a sharp incline to an elevated area. Adult birds have been observed climbing slopes as great as over 90 degrees and young hatchlings only a couple of days old able to climb slopes of 45 degrees. Through this technique, WAIR causes lift from wings functioning as thrust to accelerate the body toward the slope surface being climbed, thereby increasing friction and aiding the feet in gaining purchase, or better hindlimb traction (Tobalske and Dial).

Wing strokes creating aerodynamic forces are the main power used to direct the bird in the direction of travel. Each bird alters their normal transversely oriented wing-beat stroke toward a more anteroposterior plane directing the body downward towards the feet to counter gravitational torque on the center of mass (COM) being pulled down the slope. The end result is the wings holding the animal’s feet against the sloped substrate, aiding traction (Dial, Randall and Dial,2006). It has even been found that partially developed wings are able to produce significant and functional aerodynamic forces with early symmetrically constructed feathers. The forces created are only assisting climbing and upon descent also, help to lower impact speed (Dial, Jackson and Segre,2008)

Limiting factors to this process are traction and center of mass. Traction is defined as foot claws in contact with the substrate. This becomes limiting when the angle of the incline becomes greater than 45 degrees (Dial, Randall and Dial,2006); texture and incline. The results of tests by Dial showed that the degree of traction of the substrate is an important factor when observing the performance of WAIR when going to elevated refuge (Dial,2003). As well, the higher an incline is, the stronger the forces exerted are and a stronger anteroposterior component is present (Dial,2003). The center of mass (COM) is limiting as the animal is required to lower its posture and reorient its wing-stroke plane to avoid falling backwards down the incline. This movement provides force to counteract gravitational torque by moving the COM lower and farther forward while ensuring sufficient foot traction (Dial, Randall and Dial,2006). This is very similar to the way humans lean forward and down when going up a hill on a bike or on horseback, only birds are walking up a hill at a fast speed. This movement is emphasized during the wing-beat cycle most, as present instantaneous acceleration is suggested to be brought on during the late stages on wing down-stroke as the bird COM is accelerated towards the substrate (Bundle and Dial). This movement of the COM towards the substrate allows the animal to run vertically (Dial,2003).

Hypothesis and Evolution of Flight

The WAIR hypothesis is an alternate theory to the past traditional arboreal-cursorial dichotomy accepted by scientists today (Dial, Randall and Dial,2006). The WAIR behaviour is predicted to be a common, phylogenetically widespread activity in both basal and derived avian species representing the full altricial to precocial development spectrum (Dial, Randall and Dial, 2006). This new theory is testable and inclusive of many bird species, as demonstrated by Dial’s discovery of the behaviour through Chukar partridges, and resolves issues brought by the strict cursorial or arboreal positions of evolution. WAIR is thought to represent the intermediate stage in development of flight-capable aerodynamic wings leading to aerodynamic forces being directed toward the substrate to augment hindlimb traction and vertical movements which is then redirected to permit rudimentary aerial ascent and controlled descent (Dial, Randall and Dial,2006). Due to much morphological common characteristics of modern birds to bipedal proto-birds, it has been suggested that ancestral birds have been able to have the same locomotor advantages as present birds.

Overall, wing assisted incline running led to thoughts it is a plausible behavioural and morphological pathway for flight evolution. According to Dial, WAIR-like behaviour contains, “adaptive incremental stages that might have been exhibited by the lineage of feathered, maniraptoran dinosaurs attaining powered flight” (Dial, Randall and Dial, 2006). With this in mind, WAIR provides logical structure in which the proto-wings of avian ancestors may have offered adaptive benefits despite not having the ability of flapping aerial flight. It also demonstrates how transitional stages of wings have been adaptive, particularly in small bipedal cursors. Dial suggests that WAIR even provides a model identifying incremental adaptive plateaus by which the evolution of flight may have occurred (Bundle and Dial).

Ontogenetic-Transitional Wing Hypothesis

There has been much argument and analysis done to figure out the origin of flight and how ancestral proto-birds’ behaviour lead to present day flight behaviours. The previously accepted theories of flight have been the arboreal and cursorial models. With the introduction of the WAIR hypothesis, the theory of ontogenetic-transitional wing hypothesis(OTWH) was created and meant to provide a simple adaptive argument for the evolution of flight. The new theory embraces the most prominent features of both models yet differs as it proposes gliding to be a derived condition mostly confined to adult individuals of non-basal taxa birds (Dial, Jackson and Segre,2008). The OTWH states that incremental adaptive stages lead to evolution corresponding behaviourally and morphologically to transitional stages observed in ontogenetic forms (Dial, Jackson and Segre,2008). These transitional stages led the wing, during development, develop to represent an evolutionary transitional form. This theory is also flap-based involving an aerodynamically functional proto-wing incorporating both simultaneous and independent uses of both the legs and the wings, assuming that a fundamental wing-stroke is established for aerodynamic function early in bipedal ancestry leading to birds (Dial, Jackson and Segre, 2008).

Through his research, Dial has proposed that incipiently feathered forelimbs of small bipedal proto-birds(Heers, Tobalske and Dial) provided the same locomotor advantages for incline running as are present in extant birds thus this behaviour would be representative of an intermediate stage in development of flight capable aerodynamic wings (Dial,2003). In other words of Kenneth Dial, “As in the intermediate condition exemplified by the glenoid orientation of Archaeopteryx, the anteroposterior limb excursion employed by juvenile and adult ground birds during WAIR exhibits the humeral movement expected of a transitional stage.” (Dial,2003)

Bone structure adaptations

Anatomy of Bird Skeleton

In order to participate in powered flight, birds must be able to create enough lift to fly. Instead of increasing wing size to body size ratio, birds have evolved to make their bodies more suited to flight through a variety of evolutionary adaptations, such as changing bone structure, function and quantity. Compared to other modern vertebrates, birds have a reduced skeleton and reinforced bones needed for flight.

Bird bones have gradually evolved from the less dense bones found in reptiles to the ‘honeycombed’ (due to the loss of marrow) (Casinos and Cubo 161), more dense bones found in today’s extant Aves. By increasing the density of the bones, birds have successfully increased the strength of their bones, but decreased the mass. Birds can resist the physical stressors of flight, while not being encumbered by any extra weight.

Fossils of the extinct Archaeopteryx found that the original birds had teeth as well as a bony tail. They were still small (around the size of a jay or small gull) and had a small, cartilaginous sternum attached to the pelvis by belly ribs (Chiappe and Gareth 134-135). This would have kept their weight down, but made it more difficult to fly.

With the evolution into a more modern clade of birds Ornithurae, more specializations were made in bone structure and function. The extinct hesperornithiforms and ichthyornis were seabirds that helped lead bone evolution to modern Aves. Skull anatomy had evolved to the more resent form, including present cranial kinematic properties, as well as the evolution of fully heterocoelous vertebrae. Changes also occurred throughout the body, such as the emergence of a more robust pelvis, complete removal of teeth, an ossified sternum to attach flight muscles, and pnuematized bones (Casinos and Cubo).

In modern Aves, most flight-capable birds have a skeleton to body mass ratio to be similar to that of modern mammals. This may be because although some bird bones are pneumatized, the femurs of modern Aves are more robust than that of mammals due to the increased weight load needed to carry by bipedal birds. Wing bones are also reduced in modern birds, but a study shows that as a wing size increases, the bone becomes thinner, and doesn’t add much more weight. (Prange, Anderson, and et al 103-22)

The reduction and expansion of the modern bird skeleton also evolved to increase the ability for powered flight. The loss of teeth and creation of a keel allowed for there to be less weight, while providing more attachment sites for flight muscles. Many of these observations of evolution come from skeletons, although more modern methods of taxonomy such as molecular markers are being used today to determine the actual sequence of events.

Ventilation adaptations

respiration flow chart

Due to the high metabolic costs of powered flight, birds have needed to find a way to efficiently provide their muscles with enough oxygen to function, as well as to remove CO2 and heat. In modern Aves, this is done using a fixed lung system with nine air sacs that act as bellows to move the air over the gas exchange sites (Duncker 44-63). Their lungs are parabronchial instead of alveolar like in mammals, which allows the lungs to keep their fixed shape while still diffusing gases in air capillaries instead of alveoli. Their uncinate process allows them to simultaneously fly and breathe which allows birds to maintain the high levels of oxygen needed for flight (Tickle, Nudds, and Codd ). Breathing in extant birds is also unidirectional compared to mammals, which allows for more oxygen-rich air to pass over the gas exchange surface with minimal mixing of old and new air ("Avian Biology"). Extinct dinosaurs that are considered to be the ancestors to modern birds also show the same fixation of lungs, as well as the structures needed for one way breathing.

By studying well preserved fossils of Pterosaurs, (Claessens, O'Connor, and Unwin ) it was shown that the modern Aves lung structure was analogous to that evolved previously by the flying reptiles. It was shown that this was due to convergent gigantism to accommodate the lung structure, where birds have been able to accommodate the unidirectional breathing system in bodies of varying sizes.

The first difference found to have arisen between ancient birds and dinosaurs was the size of the nasal passage. Prehistoric Aves had larger nasal passages, where dinosaurs had narrower nasal cavities which are more similar to extant reptiles. Despite the visual similarities between dinosaur and extinct bird lungs, dinosaur lungs are thought to not have had flow through lungs. Uncinate processes were thought to be used in ventilation in extinct dinosaurs, but these structures have no use in breathing in extant Aves. There is also not any compelling evidence that dinosaurs possessed the same musculature to breathe as in birds now. Even Archaeopteryx has not been found to have similar structures to modern birds, which makes it difficult to identify the lineage of the evolution of avian lungs.

Similiarities between avian and dinosaur lung systems

Flightless birds

Flightless Birds, as the name implies, are birds that, although they once possessed it, no longer possess the ability to fly. For many species of flightless birds, it is believed that they evolved as such due to an abundance of food and/or a lack of predators.() This abundance of food and lack of predators is suspected to be the reason behind the large size of many flightless birds, although some are still small in stature. One of the distinctive features of Flightless birds is a greatly reduced, or completely absent, keel. Flightless birds which lack a keel all belong to the group Ratites, this group contains Ostriches, Emus, Cassowaries, and more.

Ratites

Until the late 2000’s, Ratites were thought to all be descended from a common flightless ancestor. In 2008 a study done at the University of Florida, in which the DNA of many different ratites, it was found that Ostriches, Emus, Cassowaries, and other members of the group Ratites didn’t “form a natural group based on their genetic makeup.” The study found that ratites typically found in and around Australia were actually much more closely related to tinamous, a south American bird which still posseses flight, than they were to Ostriches. The results of this study implicated a new theory that ratites are descended from an ancestor that crossed the water barriers of post-Gondwana, and are a result of parallel evolution rather than being different variations of a common flightless ancestor.

Adopting Flightless-ness

After the extinction of the Dinosaurs, there was a large reduction in competition for birds as well as a large increase in available resources. With new niches to fill and very little competition, as well as low predation, birds became “so plump that they became too heavy to fly, whether they wanted to or not.” With low risk of predation on the ground, some birds are theorized to have abandoned flight as a means of transportation in favor of walking. It’s theorized that more flightless birds are found in and around New Zealand because of it’s early isolation from the other continents, the ancestor of flightless birds in New Zealand flew there and found that it no longer needed to fly to survive due to a complete lack of mammalian predators and an abundance of resources. Flightless-ness is, currently, thought to have evolved independently in modern flightless species of bird.

Vulnerability

Since many Species of Flightless birds evolved independent of mammalian predators, the introduction of humans, rats, and cats, has been especially damaging to island populations of flightless birds. Many, the Dodo being a more famous one, have either been declared extinct or at risk. As with many animals, human encroachment is, and has, been a problem for many species of Flightless bird. Two examples of the vulnerability of Flightless birds to encroachment are the Dodo, and the New Zealand Moa. Both of these species went quickly extinct with the introduction of humans to their environment.

Comparative flight systems

There are three samples of convergent evolution of flight in the animal kingdom.

Insects

Insect of doom

Many species of insects have utilized flight in their evolution and are the only invertebrates know to have done it. Flight is thought to have only evolved once during the late Devonian, about 360 million years ago. The temporal scale of evolution is fairly grey due to an absence of examples in the fossil record. There are 32 orders of Insects, and the majority of those orders have wings, sometimes being inconspicuous and hidden to only appearing in one of the sexes or in juvenile/adult stages. They have several varieties of wings, and different methods of flight to use. Insect wings and flight are much more different than those found in vertebrates. The main evolutionary theory for Insect wings is that they are thought to have evolved from abdominable gills.

Mammals

Bats are the only order, Chiroptera, of the class Mammalia that can fly. There are many other species that have the ability to glide short distances. Bats represent a further evolution from historic glider precursors evident in the use of stretched skin between limbs instead of the use of feathers. The main distinction in flight mechanisms between bats and birds is that bats flap there digits, while birds flap their entire forelimbs. Bats also have Merkel cells, which are sensitive receptors found on the surface of their wings and allow the animal to interpret the flow of wind over their wings.

Pterosaurs

Pterosaurs are an extinct order of flying reptiles. They are the earliest known vertebrates to have evolved flight. They are first seen in the late Triassic, about 220 million years ago. They have a wing morphology that sits in between birds and bats, with skin stretched out to an elongated 4th digit, almost like an entire forelimb. Due to their size they also evolved to have similar lighter/stronger bone adaptations as seen in birds.