Red Crossbills Move In - From the Treetops or the North?

This winter has been a great one for seeing crossbills in Utah, and around the United States. There was a big influx of White-winged Crossbills from the north, presumably in response to a failing spruce crop this year. But just recently, the Red Crossbills (like the one above) have shown up in big numbers down to the seed feeders. What is most surprising to me is how synchronous this arrival seemed. Around Cache County and even further south around Salt Lake City, people started reporting Red Crossbills at their feeders last weekend. Within days they were at the feeders of everyone I know who keeps a feeder. What caused this big change at the feeders? Did the local cone crop suddenly run out? Has a second big influx of crossbills, this time Red Crossbills, pushed down from the north? My guess is that it is both happening at the same time. One thing I've noticed is that at the same time Red Crossbills started showing up at feeders, I started hearing different call notes from them.

Red Crossbills have as many as nine subspecies around the continent that some people think should be considered full species. Each subspecies specializes on a different kind of conifer, has a different-sounding call note, and has a different shape to the bill that is adapted for the conifer on which they feed. Our local population has a very liquid "kip" or "quip" call that is relatively soft. In the last week, I've also heard a very dry, sharply-upturned "whit" or "swit" call from a crossbill type that is normally found feeding on Douglas-firs in the Pacific Northwest. I've also recently heard another call type that I cannot identify, but it has the same upslurred "quip" as our locals but calls more loudly and harshly, with less time between notes. So, although our cone crop may be getting low, I think we also have some new Red Crossbill visitors in town.