Part (a) is a photo of a long-tailed widowbird in flight. It is almost entirely black with orange and white markings on the upper part of the wings. It has a bluish-white bill and a long tail with numerous tail feathers. Part (b) shows two partially overlapping bell-shaped frequency versus tail length plots (distribution curves). For each plot, the mean length, corresponding to the frequency peak, is marked as a vertical line. A left-to-right arrow spanning these two lines indicates a shift of the mean tail length to a larger value.