[4][5] Analysis of mitochondrial cytochrome b sequence data indicates that the crossbills and redpolls share a common ancestor and only diverged during the Tortonian (c. 8 mya, Late Miocene).

Unfortunately, the fossil record is restricted to a Late Pliocene (c. 2 mya) species, Loxia patevi, found at Varshets, Bulgaria.

While several ornithologists seem inclined to give these forms species status, no division of the American red crossbills has yet occurred.

[7] Preliminary investigations in Europe and Asia suggest an equal, if not greater, complexity, with several different call types identified;[8][9] these call types being as different from each other as from the named species of the parrot and Scottish crossbills - suggesting either that they are valid species, or else that the parrot and Scottish crossbills may not be.

[10] Currently accepted species[11] and their preferred food sources are: Originally, the chestnut-backed sparrow-lark (Eremopterix leucotis) and Pine grosbeak (Pinicola enucleator) were also classified as belonging within the genus Loxia.

The mechanism by which the bill-crossing is developed (which usually, but not always, occurs in a 1:1 frequency of left-crossing or right-crossing morphs), and what determines the direction, has hitherto withstood all attempts to resolve it.

It is very probable that there is a genetic basis underlying the phenomenon (young birds whose bills are still straight will give a cone-opening behavior if their bills are gently pressed, and the crossing develops before the birds are fledged and feeding independently), but at least in the red crossbill (the only species which has been somewhat thoroughly researched regarding this question) there is no straightforward mechanism of heritability.