Intermittent flight studies

Most species of birds do not flap their wings continuously during flight. Rather, they exhibit one of two intermittent flight patterns: Flap-gliding, and flap-bounding. Several authors have presented mathematical models in an attempt to characterize aspects of mechanical power output during intermittent flight (Lighthill 1977, Rayner 1977, 1985; Alexander 1982, Ward-Smith 1984). These authors predicted that flap-bounding, when compared to continuous flapping flight, offers an energetic savings at high speeds, whereas flap-gliding would be more efficient than continuous flapping at low speeds. However, few species of bird exhibit both types of intermittent flight, leading some authors (Goldspink 1977; Rayner 1977, 1985) to suggest that flap-bounding is a compromise between the need to maintain muscle contractions at an optimal velocity and the need to vary power output and flight speed. In addition, the primary flight muscle, the pectoralis, of many small birds is composed of a single muscle fiber type, further limiting the range of useful strain rates for these species. Thus, this "fixed-gear hypothesis" suggests that the only economical method for small birds to vary power output is to intermittently bound.

Researchers at the Flight Laboratory have found that some small birds such as Budgerigars and Starlings exhibit both types of intermittent flight, with flap-gliding being used at lower speeds, and flap-bounding at higher speeds. This suggests that many species of small birds are capable of optimizing aerodynamically their flight styles in spite of the theoretical constraints of their muscle composition.

Further research on the intermittent flight of Zebra Finches (Taenopygia guttata) has shown that these birds do, however, exhibit some 'fixed-gear' characteristics.

As flight speed increased in the wind tunnel, budgerigars that exhibited intermittent flight at all speeds (N=4) tended to flex their wings during intermittent non-flapping periods, apparently in response to increased profile drag. From Tobalske and Dial (1994).

Publications:

Tobalske, B.W. and K.P. Dial. 1994. Neuromuscular control and kinematics of intermittent flight in budgerigars (Melopsittacus undulatus). J. Exp. Biol. 187:1-18.

Tobalske, B.W. 1995. Neuromuscular control and kinematics of intermittent flight in the european starling (Sturnus vulgaris). J. Exp. Biol. 198, 1259-1273

Tobalske, B.W. 1996. Scaling of muscle composition, wing morphology, and intermittent flight behavior in woodpeckers. Auk 113(l):151-177

 

Budgerigar flap-gliding; flight speed 20 mph (.98 mb Quicktime movie)

Budgerigar flap-bounding; flight speed 25 mph (.84 mb Quicktime movie)