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Flying animals of different masses vary widely in body proportions, but the functional implications of this variation are often unclear. We address this ambiguity by developing an integrative allometric approach, which we apply here to hummingbirds to examine how the physical environment, wing morphology and stroke kinematics have contributed to the evolution of their highly specialised flight. Surprisingly, hummingbirds maintain constant wing velocity despite an order of magnitude variation in body weight; increased weight is supported solely through disproportionate increases in wing area. Conversely, wing velocity increases with body weight within species, compensating for lower relative wing area in larger individuals. By comparing inter- and intraspecific allometries, we find that the extreme wing area allometry of hummingbirds is likely an adaptation to maintain constant burst flight capacity and induced power requirements with increasing weight. Selection for relatively large wings simultaneously maximises aerial performance and minimises flight costs, which are essential elements of humming bird life history.

Bird flight is a remarkable adaptation that has allowed the approximately 10 000 extant species to colonize all terrestrial habitats on earth including high elevations, polar regions, distant islands, arid deserts, and many others. Birds exhibit numerous physiological and biomechanical adaptations for flight. Although bird flight is often studied at the level of aerodynamics, morphology, wingbeat kinematics, muscle activity, or sensory guidance independently, in reality these systems are naturally integrated. There has been an abundance of new studies in these mechanistic aspects of avian biology but comparatively less recent work on the physiological ecology of avian flight. Here we review research at the interface of the systems used in flight control and discuss several common themes. Modulation of aerodynamic forces to respond to different challenges is driven by three primary mechanisms: wing velocity about the shoulder, shape within the wing, and angle of attack. For birds that flap, the distinction between velocity and shape modulation synthesizes diverse studies in morphology, wing motion, and motor control. Recently developed tools for studying bird flight are influencing multiple areas of investigation, and in particular the role of sensory systems in flight control. How sensory information is transformed into motor commands in the avian brain remains, however, a largely unexplored frontier. Key words: Aves, comparative biomechanics, neuromuscular control, visual guidance, wing morphing. Le vol des oiseaux est une adaptation remarquable qui a permis aux quelque 10 000 especes actuelles de coloniser tous les habitats terrestres de la planete, dont les regions polaires et de haute altitude, les iles eloignees, les deserts arides et bien d'autres. Les oiseaux presentent de nombreuses adaptations physiologiques et biomecaniques pour le vol. Bien que l'aerodynamique, la morphologie, la cinematique des battements d'ailes, l'activite musculaire et le guidage sensoriel associes au vol d'oiseau soient souvent etudies de maniere independante, en realite, ces systemes sont naturellement integres. S'il y a abondance de nouvelles etudes sur ces aspects mecanistes de la biologie aviaire, moins de travaux recents se sont penches sur l'ecologie physiologique du vol des oiseaux. Nous passons en revue la recherche a l'interface des systemes utilises dans le controle du vol et abordons plusieurs themes communs. La modulation des forces aerodynamiques pour repondre a differents defis est assuree par trois principaux mecanismes, soit la vitesse des ailes autour de l'epaule, la forme a l'interieur de l'aile et l'angle d'attaque. Pour les oiseaux qui battent des ailes, la distinction entre la modulation par la vitesse et par la forme regroupe des etudes variees sur la morphologie, le mouvement des ailes et le controle moteur. Des outils recemment mis au point pour etudier le vol des oiseaux influencent plusieurs champs d'etude, notamment le role des systemes sensoriels dans le controle du vol. La transformation de l'information sensorielle en commandes motrices dans le cerveau des oiseaux demeure toutefois une frontiere largement inexploree. [Traduit par la Redaction] Mots-cles: oiseaux, biomecanique comparative, controle neuromusculaire, guidage visuel, morphing des ailes., Introduction The flying abilities of birds are impressive; casual observation of their soaring, hovering, and manoeuvring can lead one to assume that the graceful and poised motions are almost effortless. [...]

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Gliding is an efficient form of travel found in every major group of terrestrial vertebrates. Gliding is often modelled in equilibrium, where aerodynamic forces exactly balance body weight resulting in constant velocity. Although the equilibrium model is relevant for long-distance gliding, such as soaring by birds, it may not be realistic for shorter distances between trees. To understand the aerodynamics of inter-tree gliding, we used direct observation and mathematical modelling. We used videography (60-125 fps) to track and reconstruct the three-dimensional trajectories of northern flying squirrels (Glaucomys sabrinus) in nature. From their trajectories, we calculated velocities, aerodynamic forces and force coefficients. We determined that flying squirrels do not glide at equilibrium, and instead demonstrate continuously changing velocities, forces and force coefficients, and generate more lift than needed to balance body weight. We compared observed glide performance with mathematical simulations that use constant force coefficients, a characteristic of equilibrium glides. Simulations with varying force coefficients, such as those of live squirrels, demonstrated better whole-glide performance compared with the theoretical equilibrium state. Using results from both the observed glides and the simulation, we describe the mechanics and execution of inter-tree glides, and then discuss how gliding behaviour may relate to the evolution of flapping flight.