I just posted this comment on a thread in the rare birds section, in reply to reports of 'Northern' treecreepers, but thought I'd put it here to see if there's a discussion to be had:
This field identification of 'Northern Treecreeper' is surely unreliable? I know it's become commonplace, but what are the diagnostic field (or in-hand) criteria that make this 'identification' reliable?
There are no foreign ringing recoveries for Treecreeper, Svensson says the racial differences in Europe are subtle and overlapping ("mainly slight and clinal"), and Collins illustrates familiaris as most notably having a big white supercilium that distinguishes it from macrodactyla (britannica isn't illustrated, but is supposedly very similar to macrodactyla), yet if you do a quick google for Treecreeper photos in Britain it is immediately obvious that large flaring white supercilia are common.
So what are people basing these claims on, or am I missing something?