Scottish Crossbill
One fallout of Scotland gaining independence is that the UK would lose the Scottish Crossbill as its only endemic bird…
Independence, as we all know, can be a contentious issue. This month, all eyes are upon the Scottish elections as a bellwether of public opinion, not so much on the Holyrood parliament itself, but on the bigger issue of separatism, the elephant in the room north of the ‘border’.
I suspect that nobody, in this febrile political atmosphere, has given much thought to one quirky ornithological piece of fallout from any potential Scottish independence. It is this – if Scotland does indeed secede from the union, the United Kingdom will lose its only endemic bird (and a few mosses and lichens, too.) The Scottish Crossbill will become a breeding speciality of its eponymous country only. For 400 years the Scots Pine-guzzling finches have been hidden away in their coniferous enclave as citizens of Albion. Soon, perhaps, they will be separate, as uniquely Scottish residents.
The delicious irony of this potential change of status is that, as far as Scottish Crossbills themselves are concerned, there have always been what you could term separatist and unionist camps.
This is nothing political, but has everything to do with taxonomy – is the Scottish Crossbill a separate species, or is it a minor variation of the Crossbill? This debate has, in fact, been raging for years. It is an argument that has had its heated moments, albeit not quite of the Salmond/ Sturgeon vehemence. The dispute itself is somewhat arcane, but what lies behind it is fascinating real-world biology.
What is beyond dispute is that, in the forests of north-central Scotland, there are some unusual crossbills. They are fractionally larger than Crossbills aka ‘Common’ Crossbills, have some slightly different calls and, most significantly, they have thicker bills than their widespread counterparts. The thicker, deeper bill shape is related to their preference for Scots Pine cones, while the Crossbill prefers spruce. When they breed, these differences in tree preference are significant, perhaps decisive.
The divergence in these Scottish birds has been known for more than a hundred years, and they were first described as a race of Crossbill in 1904. It was only in 1980, though, that the taxonomic powers that be decided that the Scottish Crossbill should be granted the status of a separate species.
ID difficulties
However, it’s fair to say that the separation wasn’t universally accepted. After all, if you have ever been birding within the birds’ habitat in Scotland, you will already be acutely aware of the difficulties of identifying them. In the field they are virtually identical to Crossbills. They have the same sequence of plumages, from streaky brown ( juvenile) to green (female) to messy orange (young male) to brick red (adult male).
They have the same broad habitat and, so far as is known, almost identical habits. They feed in the same unique way, placing their open bill flush against the bottom of a conifer scale and then wrenching it loose by essentially closing the bill, then extracting the seed with the tongue. They also use their feet to manipulate seed cones, and to move acrobatically around the treetops.
In the case of Crossbills, about 50% of individuals have bills in which the lower mandible twists to the right of the upper,
with the same number twisting the other way. Presumably the Scottish birds show the same quirk – although maybe most Scottish birds lean to the left?
Another similarity with Crossbill is that seed production dominates the Scottish Crossbill’s breeding. Crossbills, in common with other finches, feed their nestlings principally on a sort of seed paste, which is regurgitated by the adults. The species therefore breeds when seeds are beginning to ripen, and in the case of Scottish Crossbills, this usually means February, or even late January. There can be two or three broods until midsummer, but the action starts when it is very cold indeed.
When Scottish Crossbills meet up and pair, it seems that factors such as bill shape, head size and diagnostic calls all help to keep the birds ‘pure’. So far there has never been any evidence of hybridisation with the Crossbill, despite the fact that DNA studies show very little significant difference between the forms.
Apparently, it seems that Scottish Crossbills and Crossbills rarely flock together, either. A study published in 2007 suggested that what is going on is something known as ‘assortative mating,’ in which the populations self-isolate, to use that achingly familiar term, preventing the genes of the original parent species overwhelming the specialised birds.
This and other studies also discovered that Scottish Crossbills do indeed have unique calls, especially those known as ‘excitement calls’ – (which does rather beg the question as to whether they become confused when they are relaxed for long periods, mind you).
To the human ear, Scottish Crossbill calls are barely discernibly different, but if you record them and analyse them as sonograms (printouts of pitch against time), the pattern is clear enough.
In 2009, researchers used playback and sonograms to census the Scottish Crossbill population, coming up with about 6,800 pairs. Most of these were in Moray, Banff, Nairn, Deeside and Sutherland, with hardly any in Strathspey – which is where many a birder, including me, thought we’d seen them!
Recent discovery
However, another comparatively recent discovery has further complicated the fraught situation up north. It turns out that Scottish Crossbills and Crossbills aren’t the only crossbills breeding in the remote forests of Scotland. So does a third species, the Parrot Crossbill, a bird originating from the taiga forests of Scandinavia and Russia.
This species is notably heavier than a Crossbill, with an even thicker, broader bill, and it has a greater range of divergent calls. In many ways the Scottish Crossbill is intermediate between the two: look at the bill depths – Crossbill 9.8-12.0mm,
Scottish Crossbill 11.3-13.1mm, and Parrot Crossbill 13.1-14.9mm. There is now thought to be a minute but resident population of Parrot Crossbills in the UK, estimated in 2008 to be 131 birds, all in pine woods, both native and planted.
The Parrot Crossbill, in common with the Scottish, has a preference for Scots Pine and other pines, sending it into immediate competition, and surely habitat overlap, with its near-relative.
Throwing a further taxonomic spanner
into the works, there have been some records of hybridisation between Parrot and Scottish Crossbills.
In February 2021, something happened that muddies the already rust-tinted water to the point of contamination. There was a record of a Scottish Crossbill in France on 25th February, in the middle of the breeding season, just west of Paris, apparently identified by a sound recording of its distinctive call. If this is a genuine case of vagrancy, a proof that the Scots are still more enthusiastic about Europe than much of the rest of the UK, it asks one or two crucial questions.
Up until now, Scottish Crossbills were thought to be largely resident, with some small local movements, whereas both Crossbills and Parrot Crossbills are known to make large-scale movements after breeding, known as ‘irruptions’. If Scottish Crossbills can also make large-scale movements, and even move outside the UK, are they even endemic to Britain anymore?
The truth is that nobody really knows what goes on up in the forests of Scotland and, indeed, in many parts of the world. There are many other isolated populations of crossbills elsewhere, and in the USA they have identified at least nine separate types based on calls and habitat! But are they species? Nobody seems to be sure.
Are we going to keep our own endemic bird? It seems to be on a knifeedge. Perhaps it is our fingers that we should keep crossed?