It is normally not possible to identify the age of a Great Grey Shrike in its 3rd CY.
On the other hand it can be difficult but is frequently possible to tell that a bird is in its 2nd CY (1st Summer) based on less dark flight-feathers, more barring on underparts remaining, less dark bill, etc.
1st Winter have a pale wing-bar along tips of greater coverts and underparts usually faintly barred brownish. The greater coverts are first rufous-buff, then progressively more whitish due to sun-bleaching, and finally it becomes narrower and in the end (between late Oct and May) disappears completely through wear.
1st Summer As adult, show many remnants of pale-tipped juv greater coverts before these entirely wear off. 1st S obviously still have the juvenile remiges, so very natural that they are more brown (and abraded).
1st Summer male As adult, but underparts usually faintly barred grey, and wings and tail not entirely black, more blackish-grey or dark brownish-grey.
A few birds – even in breeding season – will probably still have remnants of pale-tipped greater coverts. Norbert Lefanc saw an adult Great Grey Shrike on its nest (in France) in May or June which had still apparently juvenile primary coverts (with whitish tips).But normally, these pale tips have worn off by then.
It’s only 1st S which have slightly more brownish remiges than adults, not autumn birds in 2ndCY.
A 2nd Winter (in its 2nd CY) has just moulted, and remiges are as black (males) or blackish (females) as in adults, and white tips to greater coverts, if any, are very small.
At least: there are no absolute rules and limits in feather wear and bleaching, a few birds will always surprise you as exceptions from the rules!
Photographs © Hakan Söderman