Update: this individual has been accepted by the Danish Rarities Committee as a 1cy female Dusky Thrush.
Vagrant East Asian thrushes, especially first year birds, may cause identification problems in Europe. We know that the species interbreed but don’t know for sure the variability of ‘pure’ birds. One classic case is the species pair Dusky Thrush (Turdus eunomus) and Naumann's Thrush (T. naumanni) which are known do produce hybrids and backcrosses creating individuals which superficially may resemble one of the two species where only few character suggest introgression from the other species. The bird from Bornholm in Denmark seen in these photos is a first winter female and it resembles mostly a Dusky Thrush. However, what is natural variation of pure individuals and what are the signs of introgression? Are those red markings on the breast and flanks indicators of influence from Naumann's Thrush?
An individual seen in December 2016 in Derbyshire also had some reddish markings on the flankes as seen here, here, and here. Another 1cy female which was seen 6.-17. november 2011 in Låssa, Uppland in Sweden has been accepted by the Swedish Rarities Committee as a Dusky Thrush even though it had some reddish marking on the flanks as seen here. Furthermore, Ian Lewington has added some warm brownish and reddish feathers to the first winter female plumage of Dusky Thrush presented in the book Rare Birds of North America as seen in the plate here.