Concord Field Station

Avian Flight

(collaboration with Bret W. Tobalske, University of Portland and Ken Dial, University of Montana)

How much mechanical power does a bird require to fly? How do its flight muscles function to produce this power?  How do power requirements and the function of muscles change as a function of differences in flight behavior?

These questions are being answered by making measurements of the power output of the avian pectoralis under varying flight conditions, based on in vivo recordings of pectoralis force and length change. These recordings provide a direct measure of the work performed by the pectoralis on a cycle-by-cycle basis, which is represented by the area enclosed within the “work-loop“ during each cycle. The work/cycle times the bird's wing beat frequency, in turn, provides a measure of the muscle's mechanical power output. Because the pectoralis represents the main muscle powering flight within the wing, its mechanical power output represents a reasonable estimate of the whole animal's power requirements for flight.

A. Pectoralis power output in pigeons, cockatiels and magpies.
Our recent work has shown that the pectoralis develops force rapidly late in the upstroke in a nearly isometric fashion, subsequently shortening over a considerable range of its length (35-40%) to perform the positive work necessary for producing aerodynamic power required for flight. Measurements made in black-billed magpies flying in a wind tunnel show that power is greatest when the birds hover, decreasing rapidly to less than 50% of hovering power at speeds of 4 to 14 m/s. Only a slight increase in power is observed at fast flight speeds. Direct measurements of muscle power output in cockatiels recently match well those predicted by aerodynamic theory applied to the birds' flight kinematics. Our results also show that differences in wing design and flight style are have important effects on the power curves of different species. Magpies, for example, have a flatter power curve associated with their lower wing loading compared with cockatiels and ring-neck doves. They also do not exhibit the increased power ouputs required at faster flight speeds, presumably due to increased drag that their lower aspect ratio wings limits their top speed, compared with cockatiels and doves.

B. Scale effects on muscle power output as a function of size within related avian species.
In a related set of studies we are beginning to explore the scaling of available mechanical power relative to required mechanical power for flight in two groups of birds: columbids and anseriformes. These studies seek to understand what sets the limit for the maximum size of large flying vertebrates and what the implications are for species diversity and locomotor behavior.

C. In vivo performance of the supracoracoideus muscle (wing elevation and rotation).
Following up our work on the in vivo force-length behavior of the pectoralis, we are now developing approaches for carrying out similar measurements of muscle length and force for its major antagonist, the supracoracoideus. We are making the first of these measurements in a large pigeon breed (White Carneau). Our preliminary results indicate that the supracoracoideus generates considerable force and does significant work during the upstroke of slow to moderate speed flight. This appears to be related to the high inertial power required to accelerate the wing. Little antagonist co-activation is observed between the pectoralis and supracoracoideus, with the peak in force occuring 33% through the upstroke, not at wing turn around, which would be expected if it also functioned to decelerate the wing during the downstroke.

D. Kinematics and aerodynamics of flight as a function of speed.
We have studied the three-dimensional kinematics of the wing and body movement in cockatiels and ring-neck doves during steady forward flight. These studies are carried out using our variable speed wind tunnel with a 1.2 square meter working section. One of the questions we seek to address is whether birds use distinguishable aerodynamic gaits over different ranges of speed. These have been defined based on hypothesized changes in the circulation pattern of airflow over the wing (discontinuous vortex ring vs continuous vortex). We are also interested in how whole body accelerations relate to inertial energy changes of the bird's wing motion. These studies are a part of Ty Hedrick's research.