There is now a third crossbill species

The Cassia Crossbill, found only in one county in Idaho, is the newest North American bird species. The American Ornithological Society recently conferred species status to this member of the formerly two-species crossbill family. Evolution of a thicker beak distinguishes this bird.

The beak is thicker because the bird is the only creature there that feeds on lodgepole pine seeds. The pine has evolved thicker cone scales to protect itself from the bird. The bird has evolved a larger, stronger beak to deal with the thicker scales covering the seeds.

It is a coevolutionary arms race.