Some female Common Reed Buntings / Rohrammern (Emberiza schoeniclus) approach the visual appearance of males. This is one such bird that initially caused some confusion. But its head is blackish-brown, not black as in males and it shows extensive streaking on the breast, unlike typical spring males. The chin is pale, not black. Another feature to detect male-like female Reed Buntings are the upppertail coverts, which are grey in males and mostly brown (with some grey traces maybe) in females.
An even more extreme bird can be seen here.
edit: another or maybe the identical individual trapped later in summer at the same location.