How many kinds of Red Crossbills are there?

Via Round Robin, Cornell Lab of Ornithology

The Red Crossbill question, by which I'm referring to the multiple and often cryptic types of the nomadic Loxia curvirostra, has been one that's vexed ornithologists and birders alike for decades.  We've known for some time that populations of this enigmatic finch show significant diversity in bill size and that these populations can be identified by their call notes, but the mounting evidence that this widespread boreal bird may well consist of multiple species whose radiation is as impressive, in its way, as the far more celebrated Galapagos finches or the honeycreepers of Hawaii, has been one that has been slower to take hold. 

Dr Craig Benkman of the University of Wyoming has been pushing the boundaries of our understanding of what would likely better be referred to as the Red Crossbill complex for years.  And Round Robin, the CLO's great bird blog, offers a short summary of his most recent findings, presented at the most recent American Ornithologist's Union meeting:

Over his career, Craig has uncovered that among Red Crossbills, this remarkable feeding adaptation is even more finely specialized. Several different types of crossbills (they may be species, or near-species) each specialize on a different species of conifer—because different kinds of trees have differently sized cones and seeds. The sizes of the crossbills’ beaks closely match the cones they feed on and can efficiently pry apart the cone scales to get at the seed inside. Even the part of the beak used to crack open the conifer seeds, once they are extracted, is sized appropriately.

In western North America, where conifers come in widely varying sizes and shapes (such as ponderosa pines, lodgepole pines, Douglas-firs, and western hemlocks), this has led to the existence of multiple “types” of Red Crossbills.

It's fascinating stuff, even if it is a little intimidating from a birder's perspective.  If the Red Crossbill is eventually split into multiple species, as many ornithologists think it should be, birders looking to add one or more of the new Red Crossbills types to their list will no doubt have Craig Benkman to thank, or blame...