My understanding is that many of the Jackdaws in northern Scandinavia disperse south for the winter, with some of these individuals reaching the UK. These birds can vary (through five recognised intergrades) from having no collar, to having a very bright and obvious white collar (like the bird that Byron photographed at St David's Cathedral). On balance, the collar tends to be more pronounced in the more northerly populations (especially in the north of Finland) and weakest or absent in the south of the species' range.
In my experience, even in northern Sweden, many of the Jackdaws have weak or indiscernible collars, and I have seen pairs nest building there where one bird has a very white collar and the other has no obvious collar at all. There is no suggestion that the northern variant is a separate species, and there is no barrier to interbreeding between the various intergrades.
The individual that David Astins has reported periodically is now resident in the county, as indeed was the bird that Byron photographed. Again, for what it is worth, I suspect that the young birds that reach here without a mate (Jackdaws tend to mate for life) occasionally pair up with one of the local birds and then remain indefinitely - but this is pure conjecture of course.
