Hi Jake,
There's been several threads on crossbills, with a lot of info in them, well worth doing a search through.
The Scottish Crossbill does appear to be a distinct taxon, but at what rank (race or species) it should be treated, is very open to debate. But the fact remains that there are crossbills in Scotland which have a set of call types different from the (yes, wait for it!!!) THREE different call types of Common Crossbill (i.e. Common Crossbill is 3 taxa, not just one!) and also different from the call types of Parrot and 2-barred Crossbills.
Genetically, so far, no difference between any of these six (3 Common, Scot, Parrot, 2-bar) has yet been discovered - the range of variation between individuals of one is greater than the differences between the averages of each type. Two possible reasons for this; first it could just be, they haven't looked at the right part of their DNA yet; the second, that there is occasional gene exchange between all of them (i.e., Andrew's X, Y and Z interbreed occasionally), allowing them to show the same range of DNA. This second is thought the more likely, but it hasn't been confirmed yet (at least not that I'm aware!). They may be six species, or they may just be six races of one species. The jury is not even out yet, they're still listening to the evidence, and will be for some time I expect.
Why is Scottish regarded as closer to Parrot than Common? - two reasons. First, Scot has a feeding ecology more like that of Parrot: it feeds more on pine (and larch) than spruce; Parrot feeds almost entirely on pine, and Common largely on spruce. Though they will all use the less-favourite conifer if their favourite is having a crop failure (better to eat something second-rate, than nothing at all!)
Second, post-glacial vegetation history: pine colonised Britain about 12,000 years ago, from across the (then dry) North Sea; it is more than likely that large-billed, pine-feeding crossbills colonised with it. In Scotland, winters aren't too severe, so the cones don't get frozen rock-hard, but in Scandinavia, they do, so crossbills there need an even larger bill to deal with frozen cones (that's one theory, anyway). At the end of the ice ages, there was no spruce in northern Europe, only further south (Alps, Balkans) and east (central Russia), so no spruce-feeding crossbills anywhere in the area. Spruce later spread north & west to Scandinavia, but never reached Britain naturally (had to wait for man to introduce it). So there were almost certainly well-established pine-feeding crossbills derived from Parrots in Scotland, well before spruce came anywhere near.
The full distribution of Parrot is yet to be determined; there's large-billed crossbills in southeast Europe (Bulgaria etc) and parts of the Mediterranean, which (the ones I saw in Bulgaria) look structurally just like Parrot and (I'm told, for some in the Med), sound like it too.
Michael
PS good luck with the project - I think with all you're getting from everyone here, you'll be able to leave teacher completely behind . . . maybe, try to be gentle on him with all this university-level stuff :king: