Monday, 10 September 2007

Three-toed Woodpecker: variation in plumage

There is marked individual variation in the plumages of Three-toed Woodpeckers. Both within, and between, populations of the two races. The main clinal variations run in a north-south direction. Northern birds are generally lightly in colour and smaller in size than southern birds. Differences in the back pattern of the two European races are well known, but back patterns can vary significantly within each race. In Central Europe in particular the backs of birds of the alpinus race can vary greatly. Most typically show dark barring on the white of the back, though this barring varies from broad continuous bands, to broken dusky bands, to vague dusky smudges, to thin faint continuous or broken lines, to fine almost invisible vermiculations. A few birds may show vertical streaks or chevrons, of varying intensity, rather than horizontal barring. In addition, some birds of this race resemble nominate in that they completely lack barring on the white back panel. There are several populations that are almost intermediary in plumage between classic tridactylus and alpinus. Poland is a good country in which to study Three-toed Woodpecker plumages as both races occur. In the Bialowieza forest and rest of northeast Poland birds are regarded as tridactylus, but they differ from Scandinavian birds (which are the same race) in several respects. For example, they are darker and have heavy black spotting on the white back panel. In fact, in most of the features and darkness of plumage they resemble alpinus. They most resemble Scandinavian birds in terms of size and in having dark flecks and spots on the flanks and belly rather than bars. In some areas of the Carpathians (in southern Poland, Slovakia and Romania) and in the Alps (in southern Germany) alpinus occurs but some birds do not show the barred backs. Locally, most birds can lack barring on the white panel on their backs. Sometimes birds show single blackish spots as they often do in northeast Poland. Other birds may have the upper part of the white back panel bordered by a dark horizontal stripe. All in all, it is not unusual to see alpinus race Three-toed Woodpeckers in the Carpathians which more closely resemble the northern European tridactylus race in back pattern. In Austria, Romania, the Czech Republic and Slovakia I have observed alpinus birds with the classic densely barred flanks and under-parts of that race but which showed only faint grey barring on a clean white back panel. It might be the case that inter-grades between the two European races occur, though there is no firm evidence to support this. Perhaps the large distances that Three-toed Woodpecker sometimes travels, when in search of winter foraging, means that the two races encounter each other more often than their mapped geographical ranges may suggest? Birds of the tridactylus race in Fenno-Scandia and European Russia periodically move to the west and south and it is reasonable that it is these birds which enter the range of the more sedentary alpinus race rather than the other way round, if indeed the two sub-species do meet. The forests of the Carpathians are located between the main distribution zones of the two races and may be a zone of contact. Or perhaps atypically marked birds are relics from a time when the range of the species stretched right through from northern to southern Europe? The very interesting photo above was taken at Schwarzer Grat, Lkr. Ravensburg, SW Germany on 24.06.2007 (Ralph Martin). It shows two females, one a classic alpinus (on the right), the other a non-typical bird with an unbarred white back (on the left) which resembles the northern nominate race.

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