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.