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Form and Function in Avian Flight
Abstract
Flapping flight is a highly effective form of locomotion which has permitted the radiation of birds into a wide range of niches. In this chapter I explore how the mechanics of flapping flight have molded the flight adaptations of birds. The paper has three main threads. First, I describe recent theoretical and experimental studies on flapping flight aerodynamics and demonstrate how the mechanical requirements of locomotion are reflected in wingbeat kinematics, in vortex wake structure, and in the action of the pectoral musculature. Next, I consider how flight performance varies with size; scaling has become a central tool in the analysis of flight in birds and has proved a useful means of predicting how different mechanical, physiological, and ecological parameters change in importance with size, morphology, and behavior. However, scaling is frequently misinterpreted: it is size-dependence of the constraints on adaptation which lead to allometric consistency in avian flight morphology, and many of these constraints can be related directly to flight mechanics. Finally, I use a multivariate analysis of wing morphology to demonstrate how these constraints interact to different degrees in different birds and underlie correlations among flight morphology, ecology, and behavior. These threads are then brought together in a discussion of the conjectural relationships between fitness and the evolution of specializations in flight morphology.
... Morphospace defined by the relationship between wing loading and aspect ratio. These two standard morphometrics are known to determine flight performance parameters and soaring-flapping mode [18,19]. The common cranes (blue) are positioned in the morphospace near obligatory soarers such as vultures and obligatory flappers such as geese (morphological data in electronic supplementary material, appendix, table S1). ...
... Photo: R. Nathan. Adapted from [18,19]. ...
Thermal soaring conditions above the sea have long been assumed absent or too weak for terrestrial migrating birds, forcing obligate soarers to take long detours and avoid sea-crossing, and facultative soarers to cross exclusively by costly flapping flight. Thus, while atmospheric convection does develop at sea and is used by some seabirds, it has been largely ignored in avian migration research. Here, we provide direct evidence for routine thermal soaring over open sea in the common crane, the heaviest facultative soarer known among terrestrial migrating birds. Using high-resolution biologging from 44 cranes tracked across their transcontinental migration over 4 years, we show that soaring performance was no different over sea than over land in mid-latitudes. Sea-soaring occurred predominantly in autumn when large water-air temperature difference followed mid-latitude cyclones. Our findings challenge a fundamental migration research paradigm and suggest that obligate soarers avoid sea-crossing not due to the absence or weakness of thermals but due to their low frequency, for which they cannot compensate with prolonged flapping. Conversely, facultative soarers other than cranes should also be able to use thermals over the sea. Marine cold air outbreaks, imperative to global energy budget and climate, may also be important for bird migration.
... The wing moult is an important stage in migrants' annual cycle. The loss of wing area due to missing and/or not fully grown primaries and secondaries increases wing loading and may impair flight performance (Rayner 1988;Swaddle and Witter 1997;Hedenström 2003). Therefore, the timing of the flight feather replacement rarely coincides with such an energy-demanding stage of the annual cycle as migration (Kjellén 1994). ...
We analysed primary and secondary feather moult and fat reserves in 539 Common Snipes captured in the middle Pripyat River Valley, an important stopover site for waders in Central Europe, between 2002 and 2022. The average daily rate of feather growth was 1.89% in primaries and 2.27% in secondaries, being one of the highest documented in waders. The estimated duration of growth for a single flight feather varied from 11 to 21 days in primaries and from 8 to 11 days in secondaries. Moreover, multiple flight feathers (up to 14) were replaced simultaneously. As a result, the wing moult in Common Snipes was rapid with the mean primary moult duration estimated at 53 days (28 June–20 August) according to the Underhill–Zucchini model, and only 20 days in secondaries (31 July–20 August) based on moult estimates of individual secondaries. Hence, although secondary feathers began to grow when primary moult was already advanced, moulting of both flight feather groups was completed in most birds at almost the same time. Our study shows that Common Snipe in the middle Pripyat River Valley exhibit very rapid wing moult with large wing gaps. Fat reserves and thus body mass of Common Snipes were the lowest when the wing gap was greatest, compensating for their reduced wing area. Late and slow movement towards wintering grounds, allows them to moult rapidly at the early stage of autumn migration, which is likely to occur only in sites with abundant food resources.
... For aerial flight, sufficiently large wings are required to produce enough lift, 397 whereas for aquatic flight, large wings produce large hydrodynamic drag. Based on this 398 intuition, Storer (1960) considered that the wing size in volant wing-propelled divers399 represents a compromise between those different functional demands (see alsoPennycuick, 400 1987;Rayner, 1988). The fact that most volant wing-propelled divers use half-folded wings in 401 aquatic flight (e.g.,Warham, 1996) renders intuitive support to this explanation.402 ...
Review on wing-propelled diving in birds, with compiled records of its occurrence. Provides new looks at traditionally held ideas of convergence and compromise in wing-propelled divers.
... The acute angle reduces the length of the dorsal elevator muscles (Gill, 2007), therefore decreasing the time required for the muscle to contract, allowing the wing to be uplifted faster. In extant birds, it can be clearly seen that the time required for the upstroke is shorter than the downstroke, which also experiences greater resistance as it pushes against the air (Rayner, 1988;Biewener, 2011). Except for the specialized condition in hummingbirds, the upstroke of extant birds does not generate lift (Biewener, 2011;Chin & Lentink, 2019), thus reduction of the time required to make the upstroke is beneficial to reduce altitude loss during powered flight. ...
Important transformations of the pectoral girdle are related to the appearance of flight capabilities in the Dinosauria. Previous studies on this topic focused mainly on paravians yet recent data suggests flight evolved in dinosaurs several times, including at least once among non-avialan paravians. Thus, to fully explore the evolution of flight-related avian shoulder girdle characteristics, it is necessary to compare morphology more broadly. Here, we present information from pennaraptoran specimens preserving pectoral girdle elements, including all purportedly volant taxa, and extensively compare aspects of the shoulder joint. The results show that many pectoral girdle modifications appear during the evolution from basal pennaraptorans to paravians, including changes in the orientation of the coracoid body and the location of the articulation between the furcula and scapula. These modifications suggest a change in forelimb range of motion preceded the origin of flight in paravians. During the evolution of early avialans, additional flight adaptive transformations occur, such as the separation of the scapula and coracoid and reduction of the articular surface between these two bones, reduction in the angle between these two elements, and elongation of the coracoid. The diversity of coracoid morphologies and types of articulations joining the scapula-coracoid suggest that each early avialan lineage evolved these features in parallel as they independently evolved more refined flight capabilities. In early ornithothoracines, the orientation of the glenoid fossa and location of the acrocoracoid approaches the condition in extant birds, suggesting a greater range of motion in the flight stroke, which may represent the acquisition of improved powered flight capabilities, such as ground take-off. The formation of a new articulation between the coracoid and furcula in the Ornithuromorpha is the last step in the formation of an osseous triosseal canal, which may indicate the complete acquisition of the modern flight apparatus. These morphological transitions equipped birds with a greater range of motion, increased and more efficient muscular output and while at the same time transmitting the increased pressure being generated by ever more powerful flapping movements in such a way as to protect the organs. The driving factors and functional adaptations of many of these transitional morphologies are as yet unclear although ontogenetic transitions in forelimb function observed in extant birds provide an excellent framework through which we can explore the behavior of Mesozoic pennaraptorans.
... Herring gulls are also regularly observed in the west area and are known to use thermal soaring over sea (Woodcock 1940b) and over land . Other species observed in the study areas, such as great cormorant Phalocrocorax carbo (west area) and blacklegged kittiwake Rissa tridactyla (north area) are not established in the literature to use thermal soaring, al though they may have the capacity to do so (Rayner 1988). Discrepancies between circling proportions in GPS and radar on the same day might indicate the presence of other species capable of thermal soaring. ...
Seabirds use several flight modes at sea, including thermal soaring, in which thermal uplift is used to gain altitude and save energy. An increase in flight altitude may have consequences for wind farm interactions if it results in birds spending more time within the rotor-swept zone (RSZ). To understand conditions under which thermal soaring occurs and potential implications for wind farm interactions, we investigated thermal soaring in relation to atmospheric conditions in June and July at 2 study areas in the North Sea, west and north of the Dutch coast. We developed algorithms that identified thermal soaring in GPS tracks of lesser black-backed gulls Larus fuscus and radar tracks of seabirds. By combining species-specific 3-dimensional information on flight behaviour from bio-logging with the continuous spatiotemporal coverage of radar positioned at wind parks, we obtained a more comprehensive overview of thermal soaring at sea than either method would obtain alone. Our results showed that birds flew at higher altitudes during thermal soaring than non-soaring flight, increasing the proportion of flight time within the RSZ. Thermal soaring occurred inside offshore wind farms to a similar degree as outside. Thermal soaring was positively correlated with positive temperature differences (ΔT) between sea surface and air (at 2 m above sea level), and north and north-westerly winds. We show that the probability of thermal soaring over the North Sea, inside and outside wind farms, increases with larger temperature differences, resulting in increased time spent within the RSZ and an increased collision risk for seabirds.
The expansion of the electric grid is inevitable. Renewable energy is on the rise, and new transmission lines must be built to link new electricity production facilities with the local network. In addition, higher electricity demand due to electrification will lead to the growth of the distribution grid. However, further construction of power lines will affect the local biodiversity. Birds are especially vulnerable: every year, power lines cause the deaths of hundreds of millions of birds by collision and electrocution. Yet the environmental impacts of the electric grid in life cycle assessment (LCA) are limited to a few impact categories, failing to cover the area of protection for damages to ecosystem quality. We developed the first methodology to quantify power lines' collision and electrocution impacts on bird richness within LCA. We calculated the potentially disappeared fraction of species (PDF) by developing species–area relationships using high‐resolution species distribution maps, species‐specific characteristics, and the location of power lines and pylons. We applied our models to Norway, a country that aims to become a low‐emission nation by 2050. The characterization factors ranged between 8.48 × 10⁻¹⁶ and 5.6 × 10⁻¹⁵ PDF*yr/kWh for collision and 3.27 × 10⁻¹⁸ and 1.66 × 10⁻¹⁶ PDF*yr/kWh for electrocution. Integrating power lines’ impacts on biodiversity in LCA is essential, as harmonized models can estimate the effects of electricity production alongside the impacts of electricity distribution. This brings us a step further in promoting a holistic assessment of energy systems.
The early period of ontogeny is key to understanding the patterns of body plan formation in birds. Most studies of avian development have focused on the development of individual avian characters, leaving their developmental integration understudied. We explored the dynamics and integration of relative percentage increments in body mass, lengths of head, skeletal elements of wing and leg, and primary flight feathers in the embryonic and postnatal development of the Rook ( Corvus frugilegus ). The relative percentage increments were calculated according to Brody's equation. Groups of similar growing traits (modules) were determined using hierarchical cluster analysis, and the degree of correlation between modules was estimated by PLS analysis. The embryonic and postnatal periods demonstrate significant consistency both in the dynamics of changes in relative percentage increments of studied traits as well as in the clustering of individual modules. The modules mainly include the body mass and head length, as well as the elements that form the fore‐ and hind limbs. Differences were revealed in the combination of modules into clusters in embryonic and postnatal periods. Hind limb elements clustered together with wing elements in the embryonic period but with body mass and the head in the postnatal period. The strongest modularity was noted for the leg in embryogenesis, and for the wing in postnatal development. The forelimb and especially the primary feathers had more distinctive growth patterns. We suggest the changes in the degree of integration between locomotor modules in ontogenesis are connected with the earlier functioning of the legs in the postnatal period and with the preparation of the wings for functioning after a chick leaves the nest.
Feathers are structures unique to birds that serve important functions such as flight, thermoregulation, and communication. Bacteria that live on the feathers, particularly ones that can break down keratin, have the potential to damage feathers and disrupt their use in communication. We predicted that birds could behaviorally manage their feather bacterial abundances by preening their feathers. We also predicted that individuals with lower feather bacterial abundances would have brighter and more colorful feathers. To test these predictions, we measured the amount of time individuals in a colony of captive Indian peafowl Pavo cristatus spent preening their feathers. We also collected feathers to determine bacteria abundance on the feather surface and to measure feather coloration. We found that birds had lower feather bacteria levels when they spent more time preening their own feathers, but only in female birds. We also found that bacteria abundances were not correlated with any feather color variables we measured. These results suggest that birds can manage feather bacterial abundances by preening but feather bacteria may not influence feather coloration in this species.
Bird wings vary in size and morphology in terms of both size and number of feathers and the underlying skeletal anatomy. The number of primary remiges does not seem to vary much between bird species but, by contrast, the number of secondary remiges is reported to range between 6 and 40 depending on bird size. Given that the primaries are attached to the manus, and the secondaries are attached to the ulna, it was predicted that as bone lengths increased with increasing size of the bird, then feather count would increase. Data were collected for 268 species from 25 different orders, and phylogenetically controlled analysis explored the allometry between feather count and bone size. The number of primaries was typically 10 or 11 and did not vary with manus size. By contrast, the number of secondaries increased with ulna length, but only in some orders. For example, in Gruiformes, the number of secondary feathers increased concomitantly with ulna length but despite a two orders of magnitude range in body mass, almost all species in the Passeriformes had nine secondary remiges. It is unclear why, for instance, species with an ulna length of 70 mm can have between 9 and 24 secondaries depending on their order. This variation in secondary feather number can be added to variation in relative wing bone lengths, flight feather lengths, flight feather mechanical properties, and flight feather vane densities as another potential mechanism of adaptation to flight requirements. The apparent constraint of wingspan is scaling as approximately body mass 1/3 . Further research is needed to explore whether changes in secondary feather number relative to ulna length are accompanied by changes in feather vane width or the overlap of adjacent feathers and how this relates to wing aerodynamics.
The kinematics, aerodynamics, and energetics of Plecotus auritus in slow horizontal flight, 2-35 m s-1, are analysed. At this speed the inclination of the stroke path is ca. 58 degrees to the horizontal, the stroke angle ca. 91 degrees, and the stroke frequency ca. 11-9 Hz. A method, based on steady-state aerodynamic and momenthum theories, is derived to calculate the lift and drag coefficients as averaged over the whole wing the whole wing-stroke for horizontal flapping flight. This is a further development of Pennycuick's (1968) and Weis-Fogh's (1972) expressions for calculating the lift coefficient. The lift coefficient obtained varies between 1-4 and 1-6, the drag coefficient between 0-4 and 1-2, and the lift:drag ratio between 1-2 and 4-0. The corresponding, calculated, total specific mechanical power output of the wing muscles varies between 27-0 and 40-4 W kg-1 body mass. A maximum estimate of mechanical efficiency is 0-26. The aerodynamic efficiency varies between 0-07 and 0-10. The force coefficient, total mechanical power output, and mechanical and aerodynamic efficiencies are all plausible, demonstrating that the slow flapping flight of Plecotus is thus explicable by steady-state aerodynamics. The downstroke is the power stroke for the vertical upward forces and the upstroke for the horizontal forward forces.
Students of ecomorphology attempt to understand the interrelationships between morphological variation among individuals, populations, species and higher taxa, and communities and the corresponding variation in their ecology. Few biologists would deny there are differences between organisms that are related to differences in their ecology. Going beyond a naive inspection of the living world and pursuing a more scientific approach to the subject makes it more controversial. Discussion among morphologists has mainly centered on problems such as the adaptive significance of certain morphological features and by what methods one can detect adaptations. Some modern papers (Peters et al., 1971; Bock, 1977) deal with these problems in detail and provide ample discussion of the difficulties involved in the study of adaptation. Another group of studies put major emphasis on the morphological variation within communities (e.g., Karr and James, 1975; Baker, 1979). An especially interesting aspect of ecomorphology is the examination of adaptive radiation of genera within families, since genera should be both systematic as well as ecological units (Inger, 1958; lilies, 1970; Leisler, 1980).
An analysis of the structure and kinematics of the forelimbs and hindlimbs of pterosaurs, and functional analogy with recent and fossil vertebrates, supports a reappraisal of the locomotory abilities of pterosaurs. A hypothesis of structural, aerodynamic, and evolutionary differences distinguishing vertebrate gliders from fliers is proposed; pterosaurs fit all the criteria of fliers but none pertaining to gliders. The kinematics of the reconstructed pterosaur flight stroke reveal a down-and-forward component found also in birds and bats; structural features of the shoulder girdle and sternum unique to pterosaurs may be explained in light of this motion. The recovery stroke of flight was accomplished, in birdlike fashion, by a functional reversal of the action of the M. supracoracoideus by the pronounced enlargement of the acrocoracoid process, which acted as a pulley. The wing membrane was supported and controlled through a system of stiffened, intercalated fibers, which were oriented like the main structural elements in the wings of birds and bats.
The hindlimbs of pterosaurs were independent of the wing membrane, and articulated like those of other advanced archosaurs and birds, not like those of bats. The gait was parasagittal and the stance digitigrade. Because of limitations on the motion of the forelimb at the shoulder, pterosaurs could not have walked quadrupedally. However, bipedal locomotion appears to have been normal and quite sufficient in all pterosaurs. There is nothing batlike about pterosaur anatomy; on the other hand, pterosaurs bear close structural resemblances to birds and dinosaurs, to which they are most closely related phylogenetically.
A lecture or a course of lectures on ‘Aircraft’ would put approximately equal emphasis on aerodynamic, structural and power-plant aspects; whereas lectures on ‘Aerodynamics of Aircraft’ would concentrate principally on aerodynamic matters while referring to just the basic elements of what limitations are imposed by structural and power-plant considerations. Similarly this lecture on the ‘Aerodynamic Aspects of Animal Flight’ will concentrate on the aerodynamic forces, and the resulting dynamic interactions, between the movements of a flying animal relative to the air and the associated air movements; and include only brief references to fundamental limitations imposed by the strength and stiffness of the skeleton of the animal and other structural considerations, or by the power-plant capabilities of the animal’s musculature and metabolism. Equally it will give only a highly condensed account (see section on Evolution) of the biologically fundamental questions of how systems for animal flight evolved in response to environmental demands and opportunities.
