A rant about crossbills
There is some really interesting research going into these birds at the moment. Documenting the differentiation in vocal types is an important part of understanding more about them. However, I am a bit concerned at notes on this forum, and the recent description published in Auk (which was rejected by AOU), of new "species" of crossbills based on differences in song, but showing no diagnosable morphological or mtDNA differences.
The obvious "null hypothesis" is that crossbills' songs are learned. There may be a bunch of different populations which roam around as groups and possibly have small differences in bill shape and food preferences. I am not saying that this is definitely what is happening. However, the papers I have read on the topic so far have not shown that this is not what is happening. Based on distinctive observed differences in song, one could perhaps call some of these bands or populations "subspecies" based on current knowledge and people might feel that naming some of them is appropriate on that basis however.
In Homo sapiens, if you go to the border between, say, the USA and Mexico or France and Spain, there is strong differentiation in voice across the border. However, we know that this is learned and not innate. We are all the same species.
For some Tyrant-Flycatchers, there are interesting studies by Kroodsma and others. Empidonax species A has been reared in captivity by Empidonax species B or whilst frequently playing recordings of the song of Empidonax species B. Emplidonax species A does not learn the song of species B, and actually sings like a normal species A. Until someone takes a pullus or egg Crossbill of song type A into captivity and plays it songs of B whilst it grows, and we can see what its song is, there is likely to be considerable scepticism about the claims of multiple species in this group (and that includes Scottish Crossbill). This instance is different from that of various other oscines with different songs, in that for crossbills, other diagnosable differences between populations are so elusive and the one character out there might be learned rather than innate.