It has been proposed elsewhere that flap-bounding, an intermittent flight style consisting of flapping phases interspersed with flexed-wing bounds, should offer no savings in average mechanical power relative to continuous flapping unless a bird flies 1.2 times faster than its maximum range speed (Vmr). Why do some species use intermittent bounds at speeds slower than 1.2Vmr? The ‘fixed-gear hypothesis’ suggests that flap-bounding is used to vary mean power output in small birds that are otherwise constrained by muscle physiology and wing anatomy to use a fixed muscle shortening velocity and pattern of wing motion at all flight speeds; the ‘body-lift hypothesis’ suggests that some weight support during bounds could make flap-bounding flight aerodynamically advantageous in comparison with continuous flapping over most forward flight speeds. To test these predictions, we studied high-speed film recordings (300 Hz) of wing and body motion in zebra finches (Taenopygia guttata, mean mass 13.2 g, N=4) taken as the birds flew in a variable-speed wind tunnel (0–14 m s-1). The zebra finches used flap-bounding flight at all speeds, so their flight style was unique compared with that of birds that facultatively shift from continuous flapping or flap-gliding at slow speeds to flap-bounding at fast speeds. There was a significant effect of flight speed on all measured aspects of wing motion except percentage of the wingbeat spent in downstroke. Changes in angular velocity of the wing indicated that contractile velocity in the pectoralis muscle changed with flight speed, which is not consistent with the fixed-gear hypothesis. Although variation in stroke-plane angle relative to the body, pronation angle of the wing and wing span at mid-upstroke showed that the zebra finch changed within-wingbeat geometries according to speed, a vortex-ring gait with a feathered upstroke appeared to be the only gait used during flapping. In contrast, two small species that use continuous flapping during slow flight (0–4 m s-1) either change wingbeat gait according to flight speed or exhibit more variation in stroke-plane and pronation angles relative to the body. Differences in kinematics among species appear to be related to wing design (aspect ratio, skeletal proportions) rather than to pectoralis muscle fiber composition, indicating that the fixed-gear hypothesis should perhaps be modified to exclude muscle physiology and to emphasize constraints due to wing anatomy. Body lift was produced during bounds at speeds from 4 to 14 m s-1. Maximum body lift was 0.0206 N (15.9 % of body weight) at 10 m s-1; body lift:drag ratio declined with increasing air speed. The aerodynamic function of bounds differed with increasing speed from an emphasis on lift production (4–10 m s-1) to an emphasis on drag reduction with a slight loss in lift (12 and 14 m s-1). From a mathematical model of aerodynamic costs, it appeared that flap-bounding offered the zebra finch an aerodynamic advantage relative to continuous flapping at moderate and fast flight speeds (6–14 m s-1), with body lift augmenting any savings offered solely by flap-bounding at speeds faster than 7.1 m s-1. The percentage of time spent flapping during an intermittent flight cycle decreased with increasing speed, so the mechanical cost of transport was likely to be lowest at faster flight speeds (10–14 m s-1).

REFERENCES

REFERENCES
Askew
G. N.
,
Marsh
R. L.
(
1998
).
Optimal shortening velocity (V / V max) of skeletal muscle during cyclical contractions: length—force effects and velocity-dependent activation and deactivation
.
J. Exp. Biol
201
,
1527
–.
Biewener
A. A.
,
Corning
W. R.
,
Tobalske
B. W.
(
1998
).
In vivo pectoralis muscle force—length behavior during level flight in pigeons (Columba livia)
.
J. Exp. Biol
201
,
3293
–.
Brown
R. H. J.
(
1963
).
The flight of birds
.
Biol. Rev
38
,
460
–.
Corning
W. R.
,
Biewener
A. A.
(
1998
).
In vivo strains in pigeonB. W. TOBALSKE, W. L. PEACOCKANDK. P. DIAL1739 Flap-bounding flight flight feather shafts: implications for structural design
.
J. Exp. Biol
201
,
3057
–.
Csicsáky
M. J.
(
1977
).
Body-gliding in the zebra finch
.
Fortschr. Zool
24
,
275
–.
Dial
K. P.
,
Biewener
A. A.
,
Tobalske
B. W.
,
Warrick
D. R.
(
1997
).
Mechanical power output of bird flight
.
Nature
390
,
67
–.
Dial
K. P.
,
Warrick
D. R.
,
Tobalske
B. W.
,
Biewener
A. A.
(
1998
).
Power output of magpies: estimates of humeral excursion via pectoral sonomicrometry and wing kinematics
.
Am. Zool
38
,
152
–.
Hill
A. V.
(
1950
).
The dimensions of animals and their muscular dynamics
.
Sci. Prog
38
,
209
–.
Kokshaysky
N. V.
(
1979
).
Tracing the wake of a flying bird
.
Nature
279
,
146
–.
Marden
J.
(
1994
).
From damselflies to pterosaurs: how burst and sustainable flight performance scale with size
.
Am J. Physiol
266
,
1077
–.
Rayner
J. M. V.
(
1979
).
A new approach to animal flight mechanics
.
J. Exp. Biol
80
,
17
–.
Rayner
J. M. V.
(
1985
).
Bounding and undulating flight in birds
.
J. Theor. Biol
117
,
47
–.
Rosser
B. W. C.
,
George
J. C.
(
1986
).
The avian pectoralis: histochemical characterization and distribution of muscle fiber types
.
Can. J. Zool
64
,
1174
–.
Rosser
B. W. C.
,
Wick
M.
,
Waldbillig
D. M.
,
Bandman
E.
(
1996
).
Heterogeneity of myosin heavy-chain expression in fast-twitch fiber types of mature avian pectoralis muscle
.
Biochem. Cell Biol
74
,
715
–.
Spedding
G. R.
(
1987
).
The wake of a kestrel (Falco tinnunculus) in flapping flight
.
J. Exp. Biol
127
,
59
–.
Thomas
A. L. R.
(
1993
).
On the aerodynamics of birds' tails
.
Phil. Trans. R. Soc. Lond. B
340
,
361
–.
Tobalske
B. W.
(
1995
).
Neuromuscular control and kinematics of intermittent flight in European starlings (Sturnus vulgaris)
.
J. Exp. Biol
198
,
1259
–.
Tobalske
B. W.
,
Dial
K. P.
(
1994
).
Neuromuscular control and kinematics of intermittent flight in budgerigars (Melopsittacus undulatus)
.
J. Exp. Biol
187
,
1
–.
Tobalske
B. W.
,
Dial
K. P.
(
1996
).
Flight kinematics of black-billed magpies and pigeons over a wide range of speeds
.
J. Exp. Biol
199
,
263
–.
Tobalske
B. W.
,
Olson
N. E.
,
Dial
K. P.
(
1997
).
Flight style of the black-billed magpie: variation in wing kinematics, neuromuscular control and muscle composition
.
J. Exp. Zool
279
,
313
–.
Ward-Smith
A. J.
(
1984
).
Analysis of the aerodynamic performance of birds during bounding flight
.
Math. Biosc
68
,
137
–.
Ward-Smith
A. J.
(
1984
).
Aerodynamic and energetic considerations relating to undulating and bounding flight in birds
.
J. Theor. Biol
111
,
407
–.
Warrick
D. R.
(
1998
).
The turning-and linear-maneuvering performance of birds: the cost of efficiency for coursing insectivores
.
Can. J.Zool
76
,
1063
–.
Warrick
D. R.
,
Dial
K. P.
(
1998
).
Kinematic, aerodynamic and anatomical mechanisms in the slow, maneuvering flight of pigeons
.
J. Exp. Biol
201
,
655
–.
Woicke
M.
,
Gewecke
M.
(
1978
).
Kinematic and aerodynamic parameters in tethered flying siskins (Carduelis spinus, Passeres, Aves)
.
J. Comp. Physiol
127
,
123
–.
This content is only available via PDF.