A Common Crossbill irruption into the UK is now underway with 100's of birds passing through Shetland, Orkney (particularly North Ronaldsay) and the Western Hebridean Islands. A colleague is on St.Kilda just now though I can't get in touch, but I wager he will come back with tales of loadsa crossbills feeding on thrift !
Some individuals and small parties appear to be hitting the Scottish and English East coast as well and I did read reports of birds passing through (and being caught) at a Danish Bird Observatory. Fair Isle do not seem to be updating their site but I guess it must be crawling with pinemunchers just now.
Calls collected from these migrants would be extremely valuable in determing which call types are 'moving' in and how they infiltrate through the mainland when they arrive (assuming they do arrive - they may continue westwards to certain death in the Atlantic). Surprisingly, we are missing this data. Calls collected by Dougie Preston on Unst last week are interesting as these gave Fc4 calls, a call which has more or less disappeared on the mainland, certainly in NE Scotland. The dominant Common call just now is 1A, a particular variant I sometimes call 'the Parroty Common' due to the similarities of the calls with it's bigger 'cousin'. After the 2005 irruption I saw increases in call type 1B, though birds giving this combination were present in some numbers before the irruption. Thus, localised breeding may have accounted for this increase. Birds on Fair Isle or any of the Isles at this time of year are definitely migrants irrupting westwards. The calls they are giving are as important, possibly more so, as their biometrics. Also, and this is maybe just me, a variance in call structure is surely more significant in practical classification than a difference of 0.2mm in bill depth ( though the latter may have some function in foraging efficiency).
If the irruption continues I will be heading to the Northern or Western Isles myself in order to get some sound recordings. If anyone fancies tagging along and sharing the costs of a hire car, or contributing towards the costs of taking my own vehicle, do get in touch - we may be doing pioneering work !
Keep your eyes and ears peeled for crossbills particularly at coastal sites or on your patch where you haven't had any for the last year or so - they may well be all the way from Fennoscandia or even East of the Ural's !