THE FLIGHT OF BIRDS
Authors: BROWN R. H. J. 
Published: January 2008

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 resistance.

The maintenance of stability in flight can be examined theoretically and it is clear that birds operate with an essentially unstable physical system, and therefore that stable flight is only possible with continuous control. This type of system is advantageous in that it permits great manoeuvrability with little expenditure of energy.

Two possible lines of the early evolution of the flight mechanism have been proposed, one from a terrestrial cursorial ancestor and the other from an arboreal form. For physical reasons the cursorial ancestor is difficult to justify. Development from an arboreal form allows one to postulate a line of evolution with feathers arising primarily as heat insulators in association with a homoiothermal physiology, later becoming adapted for flight. This theory overcomes the difficulty that feathers used for flight can have no selective advantage for this purpose at an early stage of their evolution.