In the picture, Bieri’s technical operator is showing us the differences in the plumage of the female (in the foreground) compared to the male (below), in particular at the level of the scapular coverts.

In fact, the recognition of the sexes is possible only starting from the 10th week of life, by closely observing the scapular feathers and the median coverts of the wings: the males have only a cream-colored streak along the axis of the feather, while in the female it is also visible a pair of transversal streaks (“Lorraine cross”) with respect to the axis of the feather.