With the unprecedented influx of Red-flanked Bluetail in recent days I had my head in a book last night, it used to be a book anyway but BBi makes it all so much easier to find, I digress. Back as far as 1973 the continued western spread of the species into Finland was noted and written about by Finnish birder/scientist/legend Heimo Mikkola in BB. It is interesting that back then they were reporting that this creep west had been more pronounced in years with warm springs as these helped prolong spring migration.
With climate change perhaps still relatively unheard of HM theorised that this spread west may actually be a recolonisation of former territory that had been abandoned during the last Ice Age.
With the continued warming that has happened since that time (whatever the cause) it may then follow that we can expect an increase in the ocurrence of this most magnificent of migrants and perhaps more years where we get significant numbers.