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Biomechanics of Bird Flight



J Exp Biol. 2007 Sep 15;210(Pt 18):3135-46. 
Biomechanics of bird flight.
Tobalske BW.
Department of Biology, University of Portland, 5000
North Willamette Boulevard, Portland, OR 97203, USA.

Power output is a unifying theme for bird flight and
considerable progress has been accomplished recently
in measuring muscular, metabolic and aerodynamic power
in birds. The primary flight muscles of birds, the
pectoralis and supracoracoideus, are designed for work
and power output, with large stress (force per unit
cross-sectional area) and strain (relative length
change) per contraction. U-shaped curves describe how
mechanical power output varies with flight speed, but
the specific shapes and characteristic speeds of these
curves differ according to morphology and flight
style. New measures of induced, profile and parasite
power should help to update existing mathematical
models of flight. In turn, these improved models may
serve to test behavioral and ecological processes.
Unlike terrestrial locomotion that is generally
characterized by discrete gaits, changes in wing
kinematics and aerodynamics across flight speeds are
gradual. Take-off flight performance scales with body
size, but fully revealing the mechanisms responsible
for this pattern awaits new study. Intermittent flight
appears to reduce the power cost for flight, as some
species flap-glide at slow speeds and flap-bound at
fast speeds. It is vital to test the metabolic costs
of intermittent flight to understand why some birds
use intermittent bounds during slow flight.
Maneuvering and stability are critical for flying
birds, and design for maneuvering may impinge upon
other aspects of flight performance. The tail
contributes to lift and drag; it is also integral to
maneuvering and stability. Recent studies have
revealed that maneuvers are typically initiated during
downstroke and involve bilateral asymmetry of force
production in the pectoralis. Future study of
maneuvering and stability should measure inertial and
aerodynamic forces. It is critical for continued
progress into the biomechanics of bird flight that
experimental designs are developed in an ecological
and evolutionary context.