Confined to only 70 square-kilometers of Rocky Mountain Lodgepole Pine (Pinus contorta latifolia) forest in the South Hills and Albion Mountains in southern Idaho, the Cassia Crossbill (Loxia sinesciuris) is the first “call type” or ecotype within the North American Red Crossbill (L. curvirostra) complex to be formally designated as a distinct species. Ten call types were recognized within this complex, with the Cassia Crossbill representing Type 9 (formerly the "South Hills" Crossbill). Its common name reflects the location of the South Hills and Albion Mountains in Cassia County, Idaho. The Cassia Crossbill was assigned species status because recent studies showed that it is genetically distinct from the remaining call types, and phylogenetic studies indicated that it is monophyletic, unlike the remaining call types. That is, all Cassia Crossbills analyzed to date are more closely related to each other than they are to individuals in other call types. In addition, field studies over 6 years in the South Hills and Albion Mountains found that over 99 percent of 875 breeding crossbill pairs (most of which were Cassia Crossbills) consisted of pairs of the same call type. These field studies in combination with the genetic studies indicated that the Cassia Crossbill has been and continues to be reproductively isolated from the various call types of the Red Crossbill.
Discovered in 1996, the Cassia Crossbill has been studied ever since because of its intimate relationship with its food tree, the Lodgepole Pine. In most of the range of Lodgepole Pine, the Red Squirrel (Tamiasciurus hudsonicus) is the dominate seed predator, and outcompetes crossbills by individually harvesting and caching hundreds to thousands of pine cones each autumn. In the South Hills and Albion Mountains, however, the Red Squirrel is absent. In their absence, crossbills average about 20 times more abundant, and as a result, the pines have evolved enhanced defenses against the now locally common Cassia Crossbill. Because Cassia Crossbill preferentially feeds on cones with thinner scales, and thereby exerts selection on the Lodgepole Pine, each successive generation of the pine has thicker scales near the tips of the cones where the crossbills feed. In turn, Cassia Crossbill has evolved a deeper bill to access the seeds (the trees have also lost defenses directed at the Red Squirrel, but these changes do not influence the feeding performance of crossbills much). This coevolutionary arms race between crossbill and pine has caused divergent selection between crossbills feeding on cones in the presence and absence of Red Squirrels, favoring larger-billed crossbills in the absence of squirrels. The large differences in the cones between regions favored the Cassia Crossbill to be sedentary and range-restricted, and explains why the call types of Red Crossbill (mostly types 2 and 5) that commonly move through the South Hills and Albion Mountains rarely remain, and ultimately accounts for the recognition of Cassia Crossbill as a distinct species. Its species epithet, sinesciuris, acknowledges a key condition for its evolution: “without squirrels.”