British Birding Association UK400 Club

Yes Norman, a beautiful collection of images taken by Mark Breaks on Fair Isle today - and increasing the flock size there to four birds - an adult male and three immature/females.

 

Although the prospects for us birdwatchers seeing these birds is rather exciting, I wonder what implications the irruption has on the crossbills themselves. We can only assume that the reason so many are moving south is a complete failure in their food source - the Larch cone-crop. They also eat the seeds of the Norway Spruce but when in the barren Northern Isles of Scotland, they take to eating thistle seeds and other 'weed' seeds. A poor alternative perhaps?

 

However, on the other hand, is such an influx representative of a bumper breeding season and perhaps a bumper crop of food in its breeding area. It is interesting to note that Common Crossbills irrupted in Britain as early as May this year (rather than late June when it normally occurs) and in Northern Canada, 'Red' Crossbills do not appear to be associated with the movement at all.

 

What are the theories behind this influx that others are considering responsible?