Scottish Crossbills

I received this helpful and informative email from Keith at the RSPB when I sent in my photos so thought I would share:
Nice crossbill photos! Thanks for sending them.
I think those are common crossbills rather than Scottish crossbills.
This is based on bill size and the fact that they are using a spruce tree. I’ll explain why in a bit more detail but it’s long-winded, so bear with me.
Crossbills are a VERY complicated group of birds to get one’s head around and there is a good deal of debate about just how many different species there actually are across the world.
But we are confident that there are three different species of crossbill known to use Scottish woodlands (actually four, but we’ll come back to that). They are closely related but the main difference between them is that they have different feeding strategies adapted to different conifer species. This mainly shows in them having different bill sizes.
Scottish crossbill is very sedentary – which is itself an unusual thing for a crossbill – and is tightly restricted to Caledonian pine woods since it is highly adapted to Scots pine and probably cannot feed successfully on any other type of tree. It presumably evolved by being isolated for thousands of years in ‘islands’ of Caledonian pine forest away from contact with other Eurasian crossbill types. It has a larger bill than common crossbill (although it is fair to say that all crossbills have very large bills to start with) but there is a big overlap in bill sizes, so hard to identify by just looking at the bill. But the birds in your photos do seem to have bills at the smaller end of the scale which fits with common crossbill.
There is another pine specialist crossbill, the parrot crossbill, but this is far more globally widespread species occurring right across the western parts of northern Eurasia and is relatively mobile with occasional ‘irruptive’ movements into the UK from time to time – just like waxwings do and are more well known for doing. Some individuals from these invasions stay for extended periods and are known to breed in Scotland in small numbers. Parrot crossbills have even bigger bills than Scottish crossbills but, again, there is a lot of overlap in bill sizes, so you cannot easily tell just from looking at them.
Common crossbill is, as its name suggests, is the most widespread species, both globally and within the UK. It is highly ‘irruptive’, invading from northern Eurasia, sometimes in very large numbers and much more frequently than parrot crossbill. It is a spruce tree feeder so, since there are no native spruce species in Britain, it would originally have struggled to remain and certainly to breed in the UK after invasion years. However, we have been planting spruce species commercially for at least a hundred years, originally the Eurasian Norway spruce and now the N American Sitka spruce, so Britain’s commercial forests are places where common crossbills can exist (although their sustained numbers are probably still dependant on repeated invasions from Scandinavia and further afield).
So your crossbills’ bill size and their use of spruce, together with the location well away from Scottish crossbill’s very restricted range, all point to then being commons.
One other reliable difference between crossbill species is their calls. The problem with this is that the differences are very subtle so that only very experienced experts can tell just by listening (and even they may be stretching credibility a bit in claiming to be able to tell!) Common crossbills are usually not too difficult to identify but in places like Strathspey, where all three species are known to occur, some experts would argue that definitive crossbill ID can only be done by comparing sonographs of recordings of the calls, sonographs being graphic representations of recordings with a characteristic ‘signature’. In fact, modern digital sound recording has been one of the most useful ways of separating different crossbill populations – which may actually be different species – from each other.
I mentioned a fourth type of crossbill earlier. This species is the two-barred crossbill. Luckily this has two enormous white wing bars so it immediately looks different from other crossbill species. It is an occasional, irruptive visitor to Scotland. It is one of the smaller-billed spruce/larch feeders (it is often found in larch plantations when it turns up in the UK). But, guess what, even this species has ID complications. A few individual common crossbills turn up from time to time with big white wing bars!