Morphology, Velocity, and Intermittent Flight in Birds

Body size, pectoralis composition, aspect ratio of the wing,and forward speed affect the use of intermittent flight in birds.During intermittent non-flapping phases, birds extend theirwings and glide or flex their wings and bound. The pectoralismuscle is active during glides but not during bounds; activityin other primary flight muscles is variable. Mechanical power,altitude, and velocity vary among wingbeats in flapping phases;associated with this variation are changes in neuromuscularrecruitment, wingbeat frequency, amplitude, and gait. Speciesof intermediate body mass (35–158 g) tend to flap-glideat slower speeds and flap-bound at faster speeds, regardlessof the aspect ratio of their wings. Such behavior may reducemechanical power output relative to continuous flapping. Smallerspecies (<20 g) with wings of low aspect ratio may flap-boundat all speeds, yet existing models do not predict an aerodynamicadvantage for the flight style at slow speeds. The behaviorof these species appears to be due to wing shape rather thanpectoralis physiology. As body size increases among species,percent time spent flapping increases, and birds much largerthan 300 g do not flap-bound. This pattern may be explainedby adverse scaling of mass-specific power or lift per unit poweroutput available from flight muscles. The size limit for theability to bound intermittently may be offset somewhat by thescaling of pectoralis composition. The percentage of time spentflapping during intermittent flight also varies according toflight speed.     

Wing design and scaling of flying fish with regard to flight performance

Fin and body dimensions of six genera of flying fish (Exocoetidae) were examined to study variation in morphological parameters in relation to aerodynamics performance. The fins are modified as wings for gliding flight. Fin area and fin span increase with increasing body mass, whereas the percentage of wing area contributed by the pectoral fins and the percentage of the caudal fin area contributed by the hypocaudal lobe remain constant. The aerodynamic design of flying fish approximates the monoplane-biplane classification proposed by Breder (1930). Scaling relationships for wing loading and aspect ratio indicate that wing morphology in the Exocoetidae is more similar to birds and bats than to other gliders. The flight performance of flying fish is a high-speed glide with a relatively flat trajectory. The wing, as indicated by the aspect ratio, is designed for high lift with low drag characteristics.     

A wing-assisted running robot and implications for avian flight evolution

DASH+Wings is a small hexapedal winged robot that uses flapping wings to increase its locomotion capabilities. To examine the effects of flapping wings, multiple experimental controls for the same locomotor platform are provided by wing removal, by the use of inertially similar lateral spars, and by passive rather than actively flapping wings. We used accelerometers and high-speed cameras to measure the performance of this hybrid robot in both horizontal running and while ascending inclines. To examine consequences of wing flapping for aerial performance, we measured lift and drag forces on the robot at constant airspeeds and body orientations in a wind tunnel; we also determined equilibrium glide performance in free flight. The addition of flapping wings increased the maximum horizontal running speed from 0.68 to 1.29 m s?1, and also increased the maximum incline angle of ascent from 5.6° to 16.9°. Free flight measurements show a decrease of 10.3° in equilibrium glide slope between the flapping and gliding robot. In air, flapping improved the mean lift:drag ratio of the robot compared to gliding at all measured body orientations and airspeeds. Low-amplitude wing flapping thus provides advantages in both cursorial and aerial locomotion. We note that current support for the diverse theories of avian flight origins derive from limited fossil evidence, the adult behavior of extant flying birds, and developmental stages of already volant taxa. By contrast, addition of wings to a cursorial robot allows direct evaluation of the consequences of wing flapping for locomotor performance in both running and flying.     

Biomechanics and physiology of gait selection in flying birds

Two wing-beat gaits, distinguished by the presence or absence of lift production during the upstroke, are currently used to describe avian flight. Vortex-visualization studies indicate that lift is produced only during the downstroke in the vortex-ring gait and that lift is produced continuously in the continuous-vortex gait. Tip-reversal and feathered upstrokes represent different forms of vortex-ring gait distinguished by wing kinematics. Useful aerodynamic forces may be produced during tip-reversal upstroke in slow flight and during a feathered upstroke in fast flight, but it is probable that downstroke forces are much greater in magnitude. Uncertainty about the function of these types of upstroke may be resolved when more data are available on wake structure in different flight speeds and modes. Inferring from wing kinematics and available data on wake structure, birds with long wings or wings of high aspect ratio use a vortex-ring gait with tip-reversal upstroke at slow speeds, a vortex-ring gait with a feathered upstroke at intermediate speeds, and a continuous-vortex gait at fast speeds. Birds with short wings or wings of low aspect ratio use a vortex-ring gait with a feathered upstroke at all speeds. Regardless of wing shape, species tend to use a vortex-ring gait for acceleration and a continuous-vortex gait for deceleration. Some correlations may exist between gait selection and the function of the muscular and respiratory system. However, overall variation in wing kinematics, muscle activity, and respiratory activity is continuous rather than categorical. To further our understanding of gait selection in flying birds, it is important to test whether upstroke function varies in a similar manner. Transitions between lifting and nonlifting upstrokes may be more subtle and gradual than implied by a binomial scheme of classification.     

Hovering and intermittent flight in birds

Two styles of bird locomotion, hovering and intermittent flight, have great potential to inform future development of autonomous flying vehicles. Hummingbirds are the smallest flying vertebrates, and they are the only birds that can sustain hovering. Their ability to hover is due to their small size, high wingbeat frequency, relatively large margin of mass-specific power available for flight and a suite of anatomical features that include proportionally massive major flight muscles (pectoralis and supracoracoideus) and wing anatomy that enables them to leave their wings extended yet turned over (supinated) during upstroke so that they can generate lift to support their weight. Hummingbirds generate three times more lift during downstroke compared with upstroke, with the disparity due to wing twist during upstroke. Much like insects, hummingbirds exploit unsteady mechanisms during hovering including delayed stall during wing translation that is manifest as a leading-edge vortex (LEV) on the wing and rotational circulation at the end of each half stroke. Intermittent flight is common in small- and medium-sized birds and consists of pauses during which the wings are flexed (bound) or extended (glide). Flap-bounding appears to be an energy-saving style when flying relatively fast, with the production of lift by the body and tail critical to this saving. Flap-gliding is thought to be less costly than continuous flapping during flight at most speeds. Some species are known to shift from flap-gliding at slow speeds to flap-bounding at fast speeds, but there is an upper size limit for the ability to bound (~0.3 kg) and small birds with rounded wings do not use intermittent glides.     

Mechanics of gliding in birds with special reference to the influence of the ground effect

A model of the mechanics of gliding without loss of altitude (horizontal gliding) is developed. The model can be employed to assess the influence of the principal drag components (induced, profile and parasite drag), choice of initial and final glide velocities and height above the ground on glide distance. For birds gliding near to the ground the ground effect acts to decrease the induced drag and increase the lift to drag ratio of the wings. Minimum drag speed is reduced for birds gliding near to the ground. The model is applied to the gliding flight of the black skimmer (Rhyncops nigra). Glide distances for given initial and final velocities are significantly increased in the influence of the ground effect over out of ground effect values.     

Aerodynamics of Pteranodon

A computer program originally designed to test glider performance was adapted and used to study the flight behaviour of Pteranodon. A drag polar was determined for the membranous wing, giving a cambered plate profile. Results of the program described the straight flight performance, the turning ability and circling within thermals. Pteranodon was found to have a very low sinking speed, a similar lift/drag ratio to gliding birds, to be capable of staying aloft at extremely low speeds and a very small turning circle. The stress involved while turning was calculated and found to be low. It is suggested that a change from settled light-wind weather to more turbulent conditions could have brought about the extinction of this highly specialized animal.     

The cost of living large: comparative gliding performance in flying lizards (Agamidae: Draco)

Despite exhibiting considerable interspecific variation in body mass, flying lizards of the genus Draco are isometric in their area-mass scaling relationships and exhibit no significant compensatory variation in wing aspect ratio. Thus, larger species are expected to be relatively poor gliders, in lieu of behavioral or physiological compensation, when compared with smaller congeners. Here we tested this hypothesis by conducting gliding performance trials for 11 Draco species spanning virtually the entire size range of the genus. We considered three primary performance variables: maximum velocity adjusted for wind conditions, height lost over a standard horizontal glide distance, and glide angle. Comparative analysis confirmed that larger species are relatively poor gliders and do not compensate substantially for their higher wing loadings via either behavioral or physiological mechanisms. Flying lizards were found to exhibit substantial context-dependent variation in glide performance, with smaller species often exhibiting extensive variation in height lost and glide angle between trials. Variation also was observed in empirically derived velocity profiles, with only a subset of individuals appearing to perform equilibrium glides. Such size-dependent variation in performance has important consequences for the ecology and evolution of flying lizards and other glissant taxa.     

Permeability of the urban matrix to arboreal gliding mammals: Sugar gliders in Melbourne,Australia

Habitat corridors that facilitate functional connectivity are a fundamental component of wildlife conservation in fragmented landscapes. However, the landscape matrix separating suitable habitat is not uniformly impermeable to movement and management to increase matrix permeability could be an alternative means to maintain connectivity. Gliding mammals are particularly sensitive to fragmentation because their movements are constrained by glide distance thresholds. Populations of gliders in cities are at risk of being isolated by increasing habitat loss and urban development, yet little is known about how the urban matrix affects glider movement. Here we investigate how the level of urbanization and tree cover in the matrix influence matrix permeability to sugar gliders (Petaurus breviceps) within suburban forest reserves. Twenty‐two sugar gliders were radio‐tracked over winter and summer at four reserves. Boundary crossing behaviour was measured as the number of times each glider crossed into the matrix, and matrix permeability was determined as the maximum distance travelled by gliders into the matrix. The majority of gliders (81%) were located in the matrix at least once, and rates of boundary crossing were consistent across urbanization and tree cover levels. Matrix permeability was negatively affected by matrix urbanization, but not by matrix tree cover, and no interaction effects were found. Although distances travelled by gliders into the matrix did not exceed 180 m, they were comparable with typical movement distances by gliders in reserves. Our results demonstrate that the urban matrix can provide suitable habitat for gliding mammals to move and forage, but that increased urbanization may inhibit glider use of the matrix irrespective of tree cover. This finding has implications for conservation planning and suggests that structurally connected areas may not be used if movement behaviour is inhibited. Conversely, management of matrix permeability could be used to maintain connectivity without needing to construct physical corridors.     

Gliding flight in Chrysopelea: turning a snake into a wing

Although many cylindrical animals swim through water, flying snakes of the genus Chrysopelea are the only limbless animals that glide through air. Despite a lack of limbs, these snakes can actively launch by jumping, maintain a stable glide path without obvious control surfaces, maneuver, and safely land without injury. Jumping takeoffs employ vertically looped kinematics that seem to be different than any other behavior in limbless vertebrates, and their presence in a closely related genus suggests that gap-crossing may have been a behavioral precursor to the evolution of gliding in snakes. Change in shape of the body by dorsoventral flattening and high-amplitude aerial undulation comprise two key features of snakes' gliding behavior. As the snake becomes airborne, the body flattens sequentially from head to vent, forming a cross-sectional shape that is roughly triangular, with a flat surface and lateral "lips" that protrude ventrally on each side of the body; these may diminish toward the vent. This shape likely provides the snake with lift coefficients that peak at high angles of attack and gentle stall characteristics. A glide trajectory is initiated with the snake falling at a steep angle. As the snake rotates in the pitch axis, it forms a wide "S" shape and begins undulating in a complex three-dimensional pattern, with the body angled upward relative to the glide path. The head moves side-to-side, sending traveling waves posteriorly toward the tail, while the body (most prominently, the posterior end) oscillates in the vertical axis. These active movements while gliding are substantially different and more dynamic than those used by any other animal glider. As the snake gains forward speed, the glide path becomes less steep, reaching minimally recorded glide angles of 13°. In general, smaller snakes appear to be more proficient gliders. Chrysopelea paradisi can also maneuver and land either on the ground or on vegetation, but these locomotor behaviors have not been studied in detail. Future work aims to understand the mechanisms of production and control of force in takeoff, gliding, and landing, and to identify the musculoskeletal adaptations that enable this unique form of locomotion.     

Sinking speeds of falling and flying Aphis fabae Scopoli

Abstract. 1. The terminal velocities of freshly anaesthetized and weighed, virginoparous alatae were measured by dropping them into updrafts of known speed. For an average specimen weighing about 0.49 mg the terminal velocity was 1.78 ms^-1 with wings closed and 0.82 ms^-1 with wings fully extended horizontally.
2. Extrapolating from the known terminal velocities of falling spheres of appropriate density, it is concluded that for inertly falling insects of constant shape the terminal velocity will be substantially lower with the wings extended than with them closed for all sizes of insect and that the horizontal distance travelled during a fall will be correspondingly greater.
3. Sudden reflex immobilization and falling was sometimes observed in aphids flying in a laboratory flight chamber, and very occasionally this immobilized state was maintained for many seconds (even during handling) before recovery and renewed flight. There was no evidence of a special falling attitude ('Fall-reflexhaltung') other than a simple 'frozen flight' attitude with wings extended horizontally and legs extended fore and aft while the insect spiralled downwards, abdomen first.
4. The flying aphid's usual response to a coloured surface below it in the flight chamber was not to cease wing-beating and fall but to fly downwards, abdomen first, at speeds up to more than 0.7 m s^-1.     

Phylogenetics and ecomorphology of emarginate primary feathers

Wing tip slots are a distinct morphological trait broadly expressed across the avian clade, but are generally perceived to be unique to soaring raptors. These slots are the result of emarginations on the distal leading and trailing edges of primary feathers, and allow the feathers to behave as individual airfoils. Research suggests these emarginate feathers are an adaptation to increase glide efficiency by mitigating induced drag in a manner similar to aircraft winglets. If so, we might expect birds known for gliding and soaring to exhibit emarginate feather morphology; however, that is not always the case. Here, we explore emargination across the avian clade, and examine associations between emargination and ecological and morphological variables. Pelagic birds exhibit pointed, high‐aspect ratio wings without slots, whereas soaring terrestrial birds exhibit prominent wing‐tip slots. Thus, we formed four hypotheses: (1) Emargination is segregated according to habitat (terrestrial, coastal/freshwater, pelagic). (2) Emargination is positively correlated with mass. (3) Emargination varies inversely with aspect ratio and directly with wing loading and disc loading. (4) Emargination varies according to flight style, foraging style, and diet. We found that emargination falls along a continuum that varies with habitat: Pelagic species tend to have zero emargination, coastal/freshwater birds have some emargination, and terrestrial species have a high degree of emargination. Among terrestrial and coastal/freshwater species, the degree of emargination is positively correlated with mass. We infer this may be the result of selection to mitigate induced power requirements during slow flight that otherwise scale adversely with increasing body size. Since induced power output is greatest during slow flight, we hypothesize that emargination may be an adaptation to assist vertical take‐off and landing rather than glide efficiency as previously hypothesized.     

Parameter study of simplified dragonfly airfoil geometry at Reynolds number of 6000

Aerodynamic study of a simplified Dragonfly airfoil in gliding flight at Reynolds numbers below 10,000 is motivated by both pure scientific interest and technological applications. At these Reynolds numbers, the natural insect flight could provide inspiration for technology development of Micro UAV’s and more. Insect wings are typically characterized by corrugated airfoils. The present study follows a fundamental flow physics study (Levy and Seifert, 2009), that revealed the importance of flow separation from the first corrugation, the roll-up of the separated shear layer to discrete vortices and their role in promoting flow reattachment to the aft arc, as the leading mechanism enabling high-lift, low drag performance of the Dragonfly gliding flight. This paper describes the effect of systematic airfoil geometry variations on the aerodynamic properties of a simplified Dragonfly airfoil at Reynolds number of 6000.The parameter study includes a detailed analysis of small variations of the nominal geometry, such as corrugation placement or height, rear arc and trailing edge shape.Numerical simulations using the 2D laminar Navier-Stokes equations revealed that the flow accelerating over the first corrugation slope is followed by an unsteady pressure recovery, combined with vortex shedding. The latter allows the reattachment of the flow over the rear arc. Also, the drag values are directly linked to the vortices’ magnitude. This parametric study shows that geometric variations which reduce the vortices’ amplitude, as reduction of the rear cavity depth or the reduction of the rear arc and trailing edge curvature, will reduce the drag values. Other changes will extend the flow reattachment over the rear arc for a larger mean lift coefficients range; such as the negative deflection of the forward flat plate. These changes consequently reduce the drag values at higher mean lift coefficients.The detailed geometry study enabled the definition of a corrugated airfoil geometry with enhanced aerodynamic properties, such as range and endurance factors, as compared to the nominal airfoil studied in the literature.     

Interactive effects of body mass changes and species‐specific morphology on flight behavior of chick‐rearing Antarctic fulmarine petrels under diurnal wind patterns

For procellariiform seabirds, wind and morphology are crucial determinants of flight costs and flight speeds. During chick‐rearing, parental seabirds commute frequently to provision their chicks, and their body mass typically changes between outbound and return legs. In Antarctica, the characteristic diurnal katabatic winds, which blow stronger in the mornings, form a natural experimental setup to investigate flight behaviors of commuting seabirds in response to wind conditions. We GPS‐tracked three closely related species of sympatrically breeding Antarctic fulmarine petrels, which differ in wing loading and aspect ratio, and investigated their flight behavior in response to wind and changes in body mass. Such information is critical for understanding how species may respond to climate change. All three species reached higher ground speeds (i.e., the speed over ground) under stronger tailwinds, especially on return legs from foraging. Ground speeds decreased under stronger headwinds. Antarctic petrels (Thalassoica antarctica; intermediate body mass, highest wing loading, and aspect ratio) responded stronger to changes in wind speed and direction than cape petrels (Daption capense; lowest body mass, wing loading, and aspect ratio) or southern fulmars (Fulmarus glacialoides; highest body mass, intermediate wing loading, and aspect ratio). Birds did not adjust their flight direction in relation to wind direction nor the maximum distance from their nests when encountering headwinds on outbound commutes. However, birds appeared to adjust the timing of commutes to benefit from strong katabatic winds as tailwinds on outbound legs and avoid strong katabatic winds as headwinds on return legs. Despite these adaptations to the predictable diurnal wind conditions, birds frequently encountered unfavorably strong headwinds, possibly as a result of weather systems disrupting the katabatics. How the predicted decrease in Antarctic near‐coastal wind speeds over the remainder of the century will affect flight costs and breeding success and ultimately population trajectories remains to be seen.     

Note on the glide of a bird with wings bent downwards

This note considers the influence of the bending down of the wings of a bird on the performance of its glide. The induced drag of bent wings is compared with the induced drag of a corresponding straight wing. Numerical results are given.     

The woolly flying squirrel and gliding: does size matter?

The woolly flying squirrelEupetaurus cinereus! Thomas, 1888 is the longest sciurid and most massive mammalian glider in the world. Because of this, there has been some question about the squirrel’s gliding ability. I document three glide events performed by this species. These glide events, coupled with comparisons of glide ratios, ponderal ratios, and a log-log plot of head + body length versus body mass with other flying squirrels, demonstrates that the woolly flying squirrel, despite its size, is a capable glider and is no more robust than other flying squirrels. Predation attempts that were observed during glide events are discussed within an evolutionary context.     

Aerodynamic aspects of formation flight in birds

In formation flight each wing flies in an upwash field generated by all other wings of the formation. This leads to a reduction in flight power demand for each wing as well as for the whole formation.Methods of theoretical aerodynamics are used to calculate the flight power reduction for arbitrarily shaped flight formations with any number of birds. These methods are applied to homogeneous and inhomogeneous flight formations in which birds of the same kind or birds of different span, aspect ratio and weight may be present.The total flight power reduction of the whole formation strongly depends on the lateral distance of the wings. A longitudinal displacement of the wings in flight direction has no influence on the total flight power reduction but only on their distribution on the involved individuals. The local flight power reduction is highest in the inner parts of the formation and decreases towards the apex and towards the side edges of the formation. Small and light individuals are automatically favoured by larger and heavier birds. It is shown that some minor portion of twist is necessary to fly in a formation without a rolling moment. In addition it turns out that the optimum flight speed of a formation is slightly lower than the optimum flight speed of single individuals.     

Aerodynamic Characteristics of a Feathered Dinosaur Measured Using Physical Models. Effects of Form on Static Stability and Control Effectiveness

We report the effects of posture and morphology on the static aerodynamic stability and control effectiveness of physical models based on the feathered dinosaur, Microraptor gui, from the Cretaceous of China. Postures had similar lift and drag coefficients and were broadly similar when simplified metrics of gliding were considered, but they exhibited different stability characteristics depending on the position of the legs and the presence of feathers on the legs and the tail. Both stability and the function of appendages in generating maneuvering forces and torques changed as the glide angle or angle of attack were changed. These are significant because they represent an aerial environment that may have shifted during the evolution of directed aerial descent and other aerial behaviors. Certain movements were particularly effective (symmetric movements of the wings and tail in pitch, asymmetric wing movements, some tail movements). Other appendages altered their function from creating yaws at high angle of attack to rolls at low angle of attack, or reversed their function entirely. While M. gui lived after Archaeopteryx and likely represents a side experiment with feathered morphology, the general patterns of stability and control effectiveness suggested from the manipulations of forelimb, hindlimb and tail morphology here may help understand the evolution of flight control aerodynamics in vertebrates. Though these results rest on a single specimen, as further fossils with different morphologies are tested, the findings here could be applied in a phylogenetic context to reveal biomechanical constraints on extinct flyers arising from the need to maneuver.     

The foraging behaviour of a nectar feeding marsupial,Petaurus australis

Summary The foraging behaviour of non-flying nectar feeding mammals has been examined rarely. The exudivorous yellow-bellied glider (Petaurus australis) was observed to feed extensively (70% of the total feeding observation time) on the nectar of all species of Eucalyptus present at a site in southeastern Australia. Gliders harvested nectar, and presumably pollen also, whenever eucalypt flowers were available and selected trees with 2–3 times as many flowers as that on trees randomly selected along a transect. The abundance of flowering trees varied temporally and, at times when few flowering trees were present, gliders chose trees with fewer flowers than at times when flowering trees were abundant. When flowering trees were superabundant or scarce, there was no relationship between the number of flowers in a tree and the duration of visits by gliders. However, at intermediate levels of abundance, the amount of time a glider spent in a tree was related to the number of flowers in a tree. Gliders devoted 90% of the time outside their dens to foraging and the above relationship is suggested to reflect two foraging options which maximize net energy gain for different abundances of flowering trees. Although gliders spent considerable lengths of time in individual trees feeding, initial deposition of cross pollen when gliders first arrive in a tree may be substantial and thus, may provide significant amounts of outcrossing for these eucalypts.     

Particle-Image Velocimetry and Force Measurements of Leading-Edge Serrations on Owl-Based Wing Models

High-resolution Particle-Image Velocimetry （PIV） and time-resolved force measurements were performed to analyze the impact of the comb-like structure on the leading edge of barn owl wings on the flow field and overall aerodynamic performance. The Reynolds number was varied in the range of 40,000 to 120,000 and the range of angle of attack was 0° to 6° for the PIV and -15° to ＋20° for the force measurements to cover the full flight envelope of the owl. As a reference, a wind-tunnel model which possessed a geometry based on the shape of a typical barn owl wing without any owl-specific adaptations was built, and measurements were performed in the aforementioned Reynolds number and angle of attack： range. This clean wing model shows a separation bubble in the distal part of the wing at higher angles of attack. Two types of comb-like structures, i.e., artificial serrations, were manufactured to model the owl＇s leading edge with respect to its length, thickness, and material properties. The artificial structures were able to reduce the size of the separation region and additionally cause a more uniform size of the vortical structures shed by the separation bubble within the Reynolds number range investigated, resulting in stable gliding flight independent of the flight velocity. However, due to increased drag coefficients in conjunction with similar lift coefficients, the overall aerodynamic performance, i.e., lift-to-drag ratio is reduced for the serrated models. Nevertheless, especially at lower Reynolds numbers the stabilizing effect of the uniform vortex size outperforms the lower aerodynamic performance.