Reading through Birding World Vol.21 Number 8 Issue 260 it struck me how many of the Two-barred Crossbills in July and August influx to Shetland last year were juveniles ( in streaked plumage). The consensus was that these 'streakies' fledged in Spring 08, which according the BWP is fairly typical. There were also adult birds accompanying them and these had undergone post breeding moult (from the pictures) eg. they were in at least their second year.
On the other hand, the Orkney 'wing barred' crossbills in June and July this year to me look like birds which fledged very early this year or possibly late last year, typical of curvirostra's breeding patterns. It is unlikely that they would have such 'retarded' wing bars if they were genuine Two-Barred's in their second calendar year ? Even the streaked juvenile two barred's last year had strong white on their greater coverts. It is also unlikely that two barred crossbills that fledged in Spring 2009 would be so advanced into their post-juvenile moult as these recent Orkney curvirostra type specimens, again based on the streaked juveniles last year in July and August. Many of the birds that have been photographed recently in the Northern Isles appear to be these 'first summer' (or second year) types, exhibiting fringes on particularly the greater coverts and some on the tertials, with mottled body moult eg. males with green blotches. A good example of a wing-barred Common that even has broadening fringes towards the body ala Two-barred is HERE - still doesn't look 'right' for Two-Barred does it ?
I have been pondering where all these Common Crossbills have come from and why ? How about these scenerios, and please bear in mind I am just 'thinking out aloud' here eg. I reserve the right to change my options or be completely wrong !
1) The Common Crossbills are from either/and/or Finland, Norway, Sweden and North West Russia. A localised Larch and Spruce failure has meant birds now have to move from there as Scot's Pine will have shed most of their seeds. They are errupting from these (nearer) areas fairly recently.
2) The birds have possibly been moving from much further East or North/East for several months, foraging crops as they go, after breeding (late last year or late autumn) and there is a 'lag' effect. If they were coming from further East, then settling temporarily in areas in Finland, Sweden etc where other crossbill populations may also be present or resident, then we could assume that these would also be moving now for the same reasons - if so, there could be differences in call types, biometrics and stable isotope analysis of feathers for these different populations.
Common Crossbills normally errupt after a successful breeding season and/or coinciding with a poor spruce crop. This means there is usually a high percentage of juveniles. For example, in 2005 there appeared to be a significant proportion of streaked juveniles on rigs, Fair Isle etc.
If the 2009 irruption does not have a high incidence of streaked juveniles (which so far it doesn't seem to) then this possibly implies scenario 2) whereby regionally 'settled' birds have bred ( in the Autumn/Late Winter) and this food source has now become depleted and they are moving westwards over several months. Where they have come from is harder to establish, though stable isotope data should determine this ( to some degree). However, the calls so far appear to be only of one type predominantly 4E with only one 1B thanks to the efforts of Dougie Preston and Hugh Harrop. It would have been interesting to know what the Orkney and St.Kilda ones were. Perhaps 4E is less selected for the crops that are available where they are moving through (possibly Scots Pine which is still holding some seeds, though I have several records of 4E's using Scot's Pine in midwinter in Scotland - nothing is straightforward with crossbills !). If the call types of the influx birds are primarily of one variety then this perhaps suggests only one curvirostra type/morph/cryptic species is errupting the continent. It would be an interesting opportunity to try and understand why this is the case.
What would have been extremely interesting is if anyone had been sound recording the calls of crossbills in these continental regions over the last year or so - we basically need several LoxiaFan's in Russia, Finland, Sweden, Norway and on each of the Northern Isles ! We would then know what call types were present and also if there were any changes, and if these changes can be 'tracked' and in what directions. I would be particularly interested in comparing call types with deuterium levels in feathers maybe this could geographically map regional 'dialects' in crossbills populations within Europe as has been done in the USA. Knowing the status of the various cone crops in these areas would also be helpful.
I'll sleep on this and see what I think, but really these birds could have come from anywhere, but the significant observation here is the apparent lack of fresh juvenile birds and the call types of those birds that have been moving through Shetland.
I told you I had a feeling about this year and an irruption !