FLIGHT MECHANICS AND CONSTRAINTS ON FLIGHT PERFORMANCE

Flight is the major mediator of migration in birds. Compared to other modes of locomotion, high travel speed and low energy for given range have encouraged the evolution of long-distance migration. Flight mecha nics makes an important contribution to the study of migration, most o bviously by computing flight performance (primarily power output and f light speed). The aerodynamic properties of flapping wings impose cons traints on flight activity which are often overlooked or are poorly un derstood. It is usually taken for granted that birds can select freely from a range of flight speeds, varying wingbeat kinematics to ensure force equilibrium but this has never been tested. From the mechanical properties of muscle it can be argued that birds should be able to fly efficiently only within certain ranges of wingbeat kinematics; since kinematics must vary with speed, muscle properties represent a potenti al constraint on choice of flight speed. This paper reviews approaches to the estimation and measurement of flight energetics and discusses factors which determine wingbeat kinematics in steady cruising flight. Wingbeat frequency is used to illustrate how aerodynamic and physiolo gical constraints on flight performance interact; mechanical factors i nfluencing frequency, in particular size and flight speed, are conside red. The aerodynamics of flight with load is reviewed briefly, and the need for efficient flight with a wide range of total masses is shown to have led to the evolution of bounding flight in small passerines.