bird flight

flight muscles

Diagram showing the large fan-shaped pectoralis muscle converging to an attachment on the humerus, and the smaller supracoracoideus deep to it at the front, attaching to the top of the humerus via a tendon passing through the foramen triosseum.

There are two main flight muscles attaching to each wing. The pectoralis muscle powers the downstroke and is proportionately very large in birds (up to 35% of body weight). The supracoracoideus is much smaller and has a tendon which curves around to attach to the top of the humerus. The supracoracoideus can provide power to the upstroke if required but more usually produces a rapid rotation of the humerus at the top of the upstroke (Poore et al, 1997).

In addition there are 48 other muscles within the wing and around the shoulder which produce the wide range of movements of the wing and its feathers.

flapping flight

gull.gif (4294 bytes) flapping flight (31K bytes)

downstroke: the wing beats down and forwards, producing lift and propulsion.

upstroke: the wing flexes in towards the body (largely due to a rotation of the humerus about its long axis) and then rises and extends ready for the next downstroke. Passive aerodynamic lift provides the restoring force for the upstroke under most cruising conditions, although during takeoff the upstroke is powered.

References:
Dial, K.P., Kaplan, S.R., and Goslow Jr, G.E. (1988) A functional analysis of the primary upstroke and downstroke muscles in the domestic pigeon (Columba livia) during flight. Journal of experimental Biology, 134, 1-16.
Poore, S.O., Sanchez-Haiman, A., and Goslow Jr, G.E. (1997) Wing upstroke and the evolution of flapping flight. Nature, 387, 799-802.

For a note about the geometry of birds' wings, click here...