The intermittent flight of Zebra Finches: Unfixed gears and body lift.

Tobalske, B. W., Peacock, W. L., and Dial, K. P. Journal of Experimental Biology (In review)

 

The "fixed-gear" hypothesis predicts that small birds are constrained by their morphology so that the only means they have for maintaining efficiency while varying mechanical power output during flight is to use intermittent, flexed-wing bounds. However, empirical studies have shown that several species (body mass 30 - 150 g) tend to flap-glide at slow speeds, flap-bound at fast speeds, and vary within-wingbeat power. To extend the test of the fixed-gear hypothesis to a particularly small species (13 g), we studied wing and body kinematics of zebra finch flying in a wind tunnel at speeds from 0 to 14 m s-1, the maximum range of speeds for which the birds would fly. Using these data, we test the validity of the fixed-gear hypothesis as it pertains to particularly small flap-bounding birds. If the hypothesis is correct, there should be no significant effect of flight speed upon various wingbeat kinematics including angular velocity of the wing, which represents an indirect measure of contractile velocity in the pectoralis. We also use kinematic data to test for the presence of body lift during the bound phase. If the birds generate body lift during bounds, flap-bounding, relative to continuous flapping flight, can potentially offer energetic savings over a broad range of forward flight speeds.

 

Wing and body kinematics measured from flying Zebra Finches (Taenopygia guttata). Pronation angle (f), angle of attack (a), body angle (b), and wingtip excursion (WE).

Relationships among wingbeat frequency (WBF), wing excursion, and speed of the downstroke in the Zebra Finch at flight speeds from 0-14 ms-1. Wingbeat frequency increased with flight speed, wing excursion decreased, and wingtip downstroke speed was lowest at intermediate speeds. The variance in wing downstroke speeds indicates that the contractile velocity of the bird's pectoral muscles of the Zebra Finch varied between speeds.

Vertical acceleration during a bound was measured by taking the second derivative of a second-order polynomial equation for a curve fitted to digitized points representing the position of the bird's eye as a function of time. In this instance, a Zebra Finch flying at 6 m s-1 exhibited a downward acceleration of 8.8 m s-2 . As this acceleration is less than the 9.81 m s-2 acceleration due to gravity during free-fall, some lift must have been produced by the body during the bound.

Body angle relative to horizontal during the bounding phases of flap-bounding flight in the Zebra Finch at flight speeds from 0 - 14 m s-1. Average body angle decreased with flight speed.

Body lift and drag acting on a Zebra Finch during the bounding phases of flap-bounding flight at wind tunnel speeds from 2 - 14 m s-1. Body lift and drag both increased with increasing flight speed to reach maximum values at 10 m s-1, and then declined slightly as speed increased to 14 m s-1. Body lift to drag ratio decreased with increasing speed.

 

 

Conclusion:

As flight speed changes, the zebra finch changes the average angular velocity of its wing, which indirectly represents a change in contractile velocity in its pectoralis. In strict terms, this result does not support the fixed-gear hypothesis. However, in comparison to values for larger birds flying over a similar range of speeds, less variation in angular velocity of the wing is exhibited by the zebra finch. Thus it is incorrect to assume that within wingbeat power output is fixed for the zebra finch, but the relatively small variation prevents a rejection of the fixed-gear hypothesis.

The zebra finch produces body lift during bounds at all flight speeds from 4 to 14 m s-1. Lift : Drag ratio changes with speed, and this indicates that the zebra finch is seeking to minimize the cost of transport at moderate speeds and minimize body drag at faster speeds. The presence of body lift during bounds functions to modify the predictions of existing mathematical models of intermittent flight such that flap-bounding could offer an aerodynamic advantage during migration (Rayner 1985). At a minimum, it reduces the cost of flap-bounding at most forward flight speeds.