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: F_1,8=176.08, P<0.0001; bird 2: F_1,7=21.45, P<0.01). Values are means ± S.E.M. (N=5 flight sequences).