The crossbills, finches of the genus Loxia, are well known for the crossed tips of their bills, with which they open green conifer cones. This enables them to beat other conifer-seed eaters to the punch, most of which must wait for the cone to open naturally. Although there are more than two currently-recognized species of crossbills, there are only two types, the far northern white-winged ones, and the red ones, which are widepsread in the northern hemisphere. The red crossbill lineage has adapted to different conifer foods with differently-sized bills. It would be pointless to evolve a specialized bill size and then interbreed with birds of other bill-types. It would be better if the bill-types could tell each other apart, and avoid interbreeding, but no plumage differences have been found to distinguish the bill-types. It appears that the form of the flight call, which does covary dependably with bill-type, MAY have that function.
I heard a presentation on this subject by Jeffrey Groth at the 1983 AOU Meeting in New York; it is certainly one of the meatiest challenges in bird evolution and taxonomy. The current state of knowledge is laid out by Matthew Young and Tim Spahr at Crossbills of North America: Species and Red Crossbill Call Types . It is dated October 11, 2017. Pieplow's western guide builds on the work of Young and Spahr. The various populations are not given names. Rather, they are refered to by a "type" of flight call. This is not unlike the way Traill's Flycatchers were treated in the 1960s. This work is still very much in progress.
On August 12, 2003, I made a long video of Red Crossbills (and Barn Swallows) at the ski lodge at Willamette Pass, approximately 5,000 feet above sea level on the crest of the Cascades in Oregon. The place is always good for Red Crossbills. The complete soundtrack of the video was uploaded to eBird, and is presented below, although on my computer most of the sonogram does not appear, even though the sound streams properly. The feature sound for today is a 26-second clip which is associated with video of a flock foraging on the ground. The sounds in that clip are clearly Type 4. Type 4 birds are referred to as the Douglasfir type, for their preferred food, and are described as having medium-sized bills. To my eye, the birds in the video have quite large bills, but I am not experienced at classifying the bill-size of crossbills. Also, although Douglasfir is abundant in Oregon, it is not dominant in the immediate vicinity of Willamette Pass. A vast stand of Lodgepole Pine stretches from the pass eastward. To the west is a mixed forest of high elevation conifers. Nonetheless, the calls in the sonogram above are unequivocally Type 4.
More to come . . . .
In writing the commentary for these posts I have made extensive use of the invaluable bioacoustic resources listed below. For phylogenetic information, I often start with a web search of "Phylogeny of x," where x is an avian genus, family, or order. That is a hit-or-miss proposition. A recently-released resource that makes phylogenetic queries more systematic is the Birds of the World website from the Cornell Lab of Ornithology. If you follow the link and type the name of an avian family into the search box, you will be able to visit a home page for that family that presents the number of genera and species, and an illustration for each genus. If you subscribe, you will get more information. Also available to subscribers is the Birds of the World species accounts. Rolled out in 2020, this is currently an amalgam of the Birds of North America series, that was initiated by the American Ornithologists' Union around 1990, a recently-initiated online equivalent for Neotropical Birds, and the Handbook of Birds of the World series that was produced by Lynx Edicions, also beginning in the 90s. BNA has been hosted by the Lab of O for some time, and they recently added HBW to their portfolio. Especially useful for my purposes is the Systematics History subsection of the BNA accounts. EBird still has its separate species pages for all the birds of the world. These feature photos, recordings, range maps, and numerical eBird statistics, but little text. Overall, the abundance and availability of resources is astounding. Never has so much been available to so many for so little.