The Red Crossbill (Loxia curvirostra) has long been considered a candidate for splits into multiple spines. This holarctic bird appears in multiple forms with differently sized bills and nine distinct call notes. Despite this variation, evidence for distinct species has been lacking. Crossbills tend to be nomadic since they depend on seed cone crops that may be plentiful one year but not the next. When and where crossbills breed may vary from year to year as a result. This makes it difficult to demonstrate reproductive isolation of any one particular population.
One exception is the Red Crossbill population of Idaho's South Hills and Albion Mountains. There, the absence of Red Squirrels leaves the crossbills with little competition for pine seeds, and the cone crop is consistent from year to year. Thus an abundant seed bank has developed, allowing crossbills to retain the same territories. Banding studies have shown that the crossbills of the South Hills have far greater site fidelity and are more likely to follow a seasonal breeding pattern than other crossbill forms.
Since these crossbills are a resident population in the South Hills, they have coevolved with the local lodgepole pine variety. The local pines have evolved thicker, scalier cones to defend their seeds from crossbills. This favors crossbills with deeper bills to retrieve seeds. Comparison with nearby crossbill types shows that the South Hills population has larger bodies and distinctively deeper bills. In addition to morphological differences, the South Hills population uses the type 9 call. South Hills crossbills only rarely mated with crossbills using another call type. Differences in bill shape and call notes suggest genetic isolation from nearby Red Crossbills with other call types.
Based on these findings, the authors of the linked paper recommend recognizing the South Hills Crossbill as a new species, with the scientific name Loxia sinesciuris. (Sinesciuris means "without squirrels.") Assuming that the relevant authorities accept the paper's findings, adding this species to a life list could present an identification challenge as difficult as any Empidonax flycatcher. Here is how the paper describes its field marks:
Diagnosis. Very similar to other Red Crossbills in North America but larger (body mass and bill depth, but upper mandible relatively short) on average than other Red Crossbills currently found commonly north of Mexico (Table 1) and with distinctive vocalizations (see below; Smith and Benkman 2007). As in other Red Crossbills, males are reddish in body coloration whereas females are greenish-gray and smaller than males (Table 1; see below).In other words, they basically look the same as Red Crossbills, but you might be able to identify them by vocalizations. The paper includes sonograms of South Hill Crossbill flight calls and songs. So far I have not been able to find any online audio of type 9 call notes. A photo of a South Hills Crossbill can be found here.