A Redwing conundrum

A Redwing conundrum; how do they manage that?

Yes, we had snow all day on Friday, but that may well have been the last heavy snowfall of this spring (and time too!), and today was quite pleasant, partly sunny and +4*C. More birdsong too. I had not planned to mail again already now (it can easily become overkill), but the song of the many Redwings, the thrush Turdus iliacus, got me  again wondering and maybe somebody on the list knows the answer to my conundrum..
Redwings vary a lot in their song, and the ones here in Tromsø often sound quite different from the ones at Andersdalen, 30 km away. When in full song, the song consists of two parts, a short positive statement (and this is the variable part) and a lot of what I would almost call mumbling afterwards, only audible from nearby. This latter part can now and then be dropped, when the song is less intense. Every individual bird keeps to the same phrase all the time, and all the birds here in Tromsø sing roughly the same, but as I said quite different from Redwings elsewhere. So far so good. 
But the song here in Tromsø is not the same from year to year; some years ago all Redwings here sang an ascending ‘wirri wirri wirri’; now this phrase is out, and they sing quite differently. But also now, all the Tromsø Redwings use more or less the same phrase. 
How do they manage that? I suppose that Redwings born here probably to a large degree return here to nest after wintering further south in Europe, so that he stock possibly genetically is more homogenous that one otherwise would expect, and that might to a degree explain why ‘all sing the same song’.  But I cannot understand how they then manage to change their song phrases over the years, and still manage to all sing more or less the same. Can someone please explain that to me?
Wim Vader, Tromsø, Norway

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