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Figures
- Fig.1.
Wingbeat frequency showed a significant curvilinear relationship (quadratic function) with air speed for both bird 1 (A) and bird 2 (B) (General Linear Model, GLM; bird 1: F1,8=126.88, P<0.0001; bird 2: F1,8=84.83, P<0.0001). The equations for the fitted curves are given in Table2. The proportion of variance (r2) explained by the statistical model was extremely high for both birds (96.9% and 96.1%, respectively). Values are means ± s.e.m. (N=5 flight sequences).
- Fig.2.
The downstroke fraction of the wingbeat cycle period versus air speed for bird 1 (A) and bird 2 (B). Horizontal lines represent the fraction of 0.5 where the downstroke and upstroke fractions are equal. For both birds, the relationship between downstroke fraction and air speed was best represented by a quadratic function (GLM; bird 1: F1,8=176.08, P<0.0001; bird 2: F1,7=21.45, P<0.01). Values are means ± s.e.m. (N=5 flight sequences).
- Fig.3.
(A,B) Wingbeat amplitude (wrist amplitude, filled circles; wingtip amplitude, open squares) increased with air speed (wrist amplitude; GLM; bird 1: F1,9=276.05, P<0.0001, r2=96.5%; bird 2: F1,8=241.85, P<0.0001, r2=96.4%). The duration of the downstroke (C,D) decreased, and the downstroke angular velocity (E,F) increased with air speeds exceeding 7ms−1. A,C,E, bird 1; B,D,F, bird 2. Values are means ± s.e.m. N=4 at air speeds 11 and 13ms−1 (bird 2, D,F); all other N=5 flight sequences.
- Fig.4.
Mid-downstroke (maximum) wingspan (A,B) decreased with increasing air speed, although for bird 1 (A) this relationship was best represented by a quadratic function and for bird 2 (B), a linear function (GLM; bird 1: F1,8=12.54, P<0.01, r2=85.9%; bird 2: F1,8=63.42, P<0.0001, r2=88.8%). (C,D) Mid-upstroke (minimum) wingspan (GLM; C, bird 1: F1,9=73.37, P<0.0001, r2=89.1%; D, bird 2: F1,8=91.09, P<0.0001, r2=91.9%) and (E,F) wingspan ratio (GLM; E, bird 1: F1,9=67.61, P<0.0001, r2=88.3%; F, bird 2: F1,8 =86.72, P<0.0001, r2=91.6%) also decreased with increasing air speed. Values are means ± s.e.m. At airspeeds 11ms−1 (bird 1, A,C,E) and 10ms−1 (bird 2, B,D,F) N=4. All other N=5 flight sequences.
- Fig.5.
Upstroke pause duration for bird 1 (A) and bird 2 (B). The average length of these pauses at each air speed was calculated using only those wingbeats containing the pauses (i.e. does not include pauses of zero length). The duration of these pauses reached maximal levels between 10 and 12ms−1. Values are means ± s.e.m. (N=5).
- Fig.6.
Wingtip path of a characteristic wingbeat in lateral view (A) and rear view (B) at 4ms−1, 8ms−1 and 12ms−1. Arrows indicate the direction of movement and filled circles indicate the position of the wingtip on each frame, with 8ms between the nearest circles. The silhouettes illustrate the body posture at the upstroke/downstroke transition.
- Fig.7.
Body tilt averaged across the upstroke, downstroke and mid-stroke portions of the wingbeat cycle. For both bird 1 (A) and bird 2 (B), body-tilt angle showed a significant quadratic relationship with air speed, and decreased with increasing air speed (GLM; bird 1: F1,8=5.92, P<0.05; bird 2: F1,7=41.78, P<0.0001). Values are means ± s.e.m. (N=5).
- Fig.8.
Tail spread (A,B) decreased with increasing air speed, although for bird 1 (A) this relationship was best represented by a cubic function, and for bird 2 (B) a quartic function (GLM; bird 1: F1,7=17.61, P<0.01; bird 2: F1,5=13.05, P<0.05). Tail-angle of attack (C,D) also decreased with increasing air speed, and for both birds this relationship was best represented by a cubic function (GLM; C, bird 1: F1,7=8.36, P<0.05; D, bird 2: F1,6=6.42, P<0.05). The angle of tail spread in some of the flight sequences for bird 1 (A) was obscured: the number of flight sequences for which tail-spread angle data were obtained, therefore, varies between 1 and 5. All other N=5. Values are means ± s.e.m.
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