Three-toed Woodpecker plumages

Today, I received some photos and a question regarding Three-toed Woodpeckers in southern Germany. It all centres on the following and it's a topic that comes up every year...

There is much individual variation in the plumages of European Three-toed Woodpecker (Picoides tridactylus), especially within the central and southern European populations which are P.t.alpinus (for more on this see "Variation" on page 158 in my book WOODPECKERS OF EUROPE, and the drawings on page 157). I can say that it is not unusual to see birds in the Carpathians, Alps and Balkan ranges that have very white and largely unmarked backs, which is more typical of the more northerly distributed P.t.tridactylus race. A good feature to look for in such birds is the amount of dark (mainly black) markings (barring and flecking) on theunderparts, esp on the flanks and belly. Most of the "white-backed" alpinus show more black barring and flecking here than classic tridactylus birds. Some observers have suggested that white-backed southern birds are tridactylus birds within the range of alpinus. Some observers have even gone as far as to say that "all" birds in this area are white-backed, and therefore tridactylus, and that no birds have barred backs as alpinus are supposed to have. I know this to be view to be wrong. Classic alpinus birds with barred backs do exist. Yet, there is much to prove, and it is not impossible that the odd northern bird wanders southwards each year, but I personally believe that such white-backed birds are the result of extreme local individual variation rather than populations of tridactylus living amongst alpinus.