Continental Great Spotted Woodpeckers in mainland Britain-fact or fiction?

The wing lengths of Great Spotted Woodpeckers Dendrocopos major resident in Britain (subspecies anglicus) are more variable than had previously been realised, with one individual in a thousand reaching extremes of 123 mm and 140 mm. As a result, many of the past British records claiming to be of the larger continental subspecies major have probably been misidentified. There is no reliable evidence to indicate that continental Great Spotted Woodpeckers visit Britain annually. Rather, infrequent irruptions into Britain occur about once in 12 years. Birds arriving at Fair Isle are at the top end of the size range of this species, and probably originate from northern Russia and Scandinavia. The birds irrupting into the mainland of Britain are intermediate in size but, on average, have longer wings than the British residents, and so only the larger individuals with wing lengths beyond the British range can be identified as of continental origin. Woodpeckers originating from continental areas south of the Baltic have similar wing lengths to British birds and cannot be identified from wing length measurements. Currently, there are only two instances of Great Spotted Woodpeckers being ringed on the European continent and recovered in Britain, again indicating that immigrant birds are probably few.

John Coulson and Nigel Odin, Ringing & Migration, 23, 217-222