During spring sexing is, in general, very straight forward due to distinct sexual dimorphism. However, a few males (2cy males only?) shows a predominantly greyish-brown upperside (instead of black) and thereby becomes more similar to females than usual.
2cy/3cy+: Most males are easily reckognized due to the black-and-white plumage and distinct white patch in the forehead, in contrast to the paler greyish-brown females (see photos below). Greyer males may be more similar to females but are still identified by (at least) traces of white in the forehead, much white in the TT and blackish uppertail-coverts. Do not use wing length.