The Flight of Birds

Abstract

Summary Bird flight can be studied neither as a problem in physics nor from the standpoint of biology alone. Both points of view are necessary and complementary. It is convenient to consider separately those birds which habitually glide or soar in air currents and those which normally fly by continuous flapping of their wings. The gliding and soaring types all obtain energy to maintain flight from air movements of various kinds. The terrestrial birds soar by making use of masses of warm air (‘thermals’) which rise from ground heated by the sun. These birds typically have large wing surfaces and fly slowly. In contrast, the gliding sea birds usually obtain energy either from air deflected upwards by cliffs or by an oceanic swell, or else they can make use of the incresea of wind velocity with height, which tends to be uniform over an unobstructed water surface. Such birds usually have long narrow wings and can glide at high speed with a small angle of descent. Within the flapping species we can distinguish four types of wing movement with different properties. First, there is the symmetrical wing flappingof the hummingbirds, which can remain stationary with the body axis vertical. Associated with this flight there are unique adaptations of both the skeleton and musculature. Secondly, there is the flapping cycle typical of the small passerine birds, where the upstroke is only a recovery stroke and takes place with the wings folded. Here, as might be expected, the elevator muscles are relatively much smaller than those of the humming birds. Thirdly, there is the flight with complex movements as seen in the pigeon, where a propulsive upstroke occurs in slow flight at take off‐and landing, but is reduced and finally disappears as the forward speed increases. Here again the relative muscle weights show adaptation, the elevators being relatively larger than those of the passerines but not as large as those found in humming birds. The fourth type, typical of large birds, shows only a simple powered downstroke and sustaining upstroke. These birds are incapable of slow flight and have small elevator muscles. The wing shape of birds is generally correlated with their type of flight, which in turn can be shown to be adapted to their habitat and mode of life. In particular, the emargination of the primary feathers may, when extensive, be either a means of improving the slow speed performance and control of land‐soaring birds, or a method of increasing the efficiency of the short broad wings of birds such as the partridge, which takes off from thickets. There have been many attempts to estimate the energy used in flight. Estimates can be made from measurements of the metabolism of the animals and also by theoretical studies of tentative aerodynamic parameters. No direct measurements of the properties of the flapping system have yet been possible. All the estimates so far made suggest both an energy output from the muscles higher than that found in non‐flying animals and also a very efficient aerodynamic system with little air ...

Aerodynamics of tip-reversal upstroke in a revolving pigeon wing

“…Given the debate over the functional significance of the tip-reversal upstroke (Brown, 1963;Spedding et al, 1984;Spedding, 1986;Aldridge, 1987;Hedrick et al, 2004;Tobalske et al, 2003), our most important result is that the model wings in upstroke posture produced a large amount of aerodynamic force (Figs4 and 5). In fact, the downstroke and upstroke wings appear to perform similarly, both reaching maximum C L :C D at approximately a26deg (Fig.5).…”

“…In contrast, wings in upstroke posture, with smaller observed s.d. at high angles of attack, may reduce these effects via the separation of the primary feathers, allowing individual feathers to function as individual airfoils with lower a (Brown, 1963). Further exploration using flow-visualization would help tease apart differences between in vivo and propeller aerodynamics.…”

“…Early flow-visualization evidence suggested that the tip-reversal upstroke is aerodynamically inactive, meaning that it does not produce lift for weight support or thrust (Spedding et al, 1984;Spedding, 1986). However, this is unlikely given dramatic differences in wing kinematics (Brown, 1953;Tobalske and Dial, 1996) between upstroke styles, and observed increases in body acceleration during the upstroke (Brown, 1963;Aldridge, 1987). These early measures of body acceleration do not account for wing inertia in calculating center of mass acceleration, and thus may not accurately reflect performance.…”

“…The first group fully flexes their wings on the upstroke and has low aspect ratio, rounded wings. The second group uses a 'tip-reversal' upstroke, in which the distal wing is heavily supinated, and includes birds with high aspect ratio, pointed wingtips (Brown, 1963;Tobalske, 2000;Tobalske et al, 2003). A notable exception to this pattern is seen in a group of birds designed for explosive take-off, Phasianidae, which have rounded, low aspect ratio wings but a prominent tip-reversal upstroke (Tobalske and Dial, 2000).…”

Behavioral State Modulates the Activity of Brainstem Sensorimotor Neurons

“…For example, gliding flight is a distinct behavioral state in pigeons that can be simulated in the laboratory with frontal airflow Bilo, 1978, 1983;Bilo, 1992Bilo, , 1994Sansonetti, 1999, 2000;Maurice and Gioanni, 2004a,b). During flight, active gaze and posture stabilization are critical, as turbulence threatens perception and performance (Brown, 1963;Erichsen et al, 1989;Warrick et al, 2002). This is reflected in state-dependent vestibular gaze-and posture-stabilizing responses (McArthur and Dickman, 2011).…”