Females of Red-backed Shrike / Neuntöter (Lanius collurio) show some variety of plumage features. While the „typical“ female has a brown head and nape with a slight grey tinge and medium (reddish) brown ear coverts, some individuals exhibit more male-like characteristics such as blackish-brown ear coverts and very grey head. I wonder if these grey-headed birds are older or if this variation is completely independent of age. Also, the amount of dark barring on the scapulars and grey on the uppertail can vary considerably.
The images below show four different females, all trapped together an photographed in the same light conditions on 16 May 2020 in Frauenkirchen, Austria.
A dark subterminal bar around the tertials has been suggested as an aging criterion (here this feature is mentioned, but at the same time its use seems to be doubted) and as a hint to sex 1st year birds. If birds showing this mark in spring (cf. bird 3, maybe bird 2?) were in their 2nd year, the amount of grey on the head wouldn’t say anything about age. According to Jenni & Winkler’s new „Moult and Ageing of European Passerines: Second Edition“, aging of 2nd year birds is not possible.